Mark more State's member function decls as inline

```
[27/110] Building CXX object test/CMakeFiles/spec_arg_verbosity_test.dir/spec_arg_verbosity_test.cc.obj
FAILED: test/CMakeFiles/spec_arg_verbosity_test.dir/spec_arg_verbosity_test.cc.obj
D:\a\_temp\msys64\clang32\bin\clang++.exe -DHAVE_STD_REGEX -DHAVE_STEADY_CLOCK -DHAVE_THREAD_SAFETY_ATTRIBUTES -DTEST_BENCHMARK_LIBRARY_HAS_NO_ASSERTIONS -ID:/a/benchmark/benchmark/include -Wall  -Wextra  -Wshadow  -Wfloat-equal  -Wold-style-cast  -Werror  -pedantic  -pedantic-errors  -Wshorten-64-to-32  -fstrict-aliasing  -Wno-deprecated-declarations  -Wno-deprecated  -Wstrict-aliasing  -Wthread-safety  -Wno-unused-variable -std=c++11 -fvisibility=hidden -fvisibility-inlines-hidden   -UNDEBUG -MD -MT test/CMakeFiles/spec_arg_verbosity_test.dir/spec_arg_verbosity_test.cc.obj -MF test\CMakeFiles\spec_arg_verbosity_test.dir\spec_arg_verbosity_test.cc.obj.d -o test/CMakeFiles/spec_arg_verbosity_test.dir/spec_arg_verbosity_test.cc.obj -c D:/a/benchmark/benchmark/test/spec_arg_verbosity_test.cc
In file included from D:/a/benchmark/benchmark/test/spec_arg_verbosity_test.cc:5:
D:/a/benchmark/benchmark/include/benchmark/benchmark.h:999:44: error: 'benchmark::State::KeepRunning' redeclared inline; 'dllimport' attribute ignored [-Werror,-Wignored-attributes]
  999 | inline BENCHMARK_ALWAYS_INLINE bool State::KeepRunning() {
      |                                            ^
D:/a/benchmark/benchmark/include/benchmark/benchmark.h:1003:44: error: 'benchmark::State::KeepRunningBatch' redeclared inline; 'dllimport' attribute ignored [-Werror,-Wignored-attributes]
 1003 | inline BENCHMARK_ALWAYS_INLINE bool State::KeepRunningBatch(IterationCount n) {
      |                                            ^
D:/a/benchmark/benchmark/include/benchmark/benchmark.h:1075:60: error: 'benchmark::State::begin' redeclared inline; 'dllimport' attribute ignored [-Werror,-Wignored-attributes]
 1075 | inline BENCHMARK_ALWAYS_INLINE State::StateIterator State::begin() {
      |                                                            ^
D:/a/benchmark/benchmark/include/benchmark/benchmark.h:1078:60: error: 'benchmark::State::end' redeclared inline; 'dllimport' attribute ignored [-Werror,-Wignored-attributes]

 1078 | inline BENCHMARK_ALWAYS_INLINE State::StateIterator State::end() {

      |                                                            ^

```

include/benchmark/benchmark.h

@@ -746,13 +746,13 @@ class BENCHMARK_EXPORT State {
   // have been called previously.
   //
   // NOTE: KeepRunning may not be used after calling either of these functions.
-  BENCHMARK_ALWAYS_INLINE StateIterator begin();
-  BENCHMARK_ALWAYS_INLINE StateIterator end();
+  inline BENCHMARK_ALWAYS_INLINE StateIterator begin();
+  inline BENCHMARK_ALWAYS_INLINE StateIterator end();
 
   // Returns true if the benchmark should continue through another iteration.
   // NOTE: A benchmark may not return from the test until KeepRunning() has
   // returned false.
-  bool KeepRunning();
+  inline bool KeepRunning();
 
   // Returns true iff the benchmark should run n more iterations.
   // REQUIRES: 'n' > 0.
@@ -764,7 +764,7 @@ class BENCHMARK_EXPORT State {
   //   while (state.KeepRunningBatch(1000)) {
   //     // process 1000 elements
   //   }
-  bool KeepRunningBatch(IterationCount n);
+  inline bool KeepRunningBatch(IterationCount n);
 
   // REQUIRES: timer is running and 'SkipWithMessage(...)' or
   //   'SkipWithError(...)' has not been called by the current thread.