Silence spurious GCC 8 warning

For an unknown reason GCC 8 thinks is_float is never used.

jbmc/src/java_bytecode/expr2java.h

@@ -58,7 +58,7 @@ std::string type2java(const typet &type, const namespacet &ns);
 template <typename float_type>
 std::string floating_point_to_java_string(float_type value)
 {
-  const auto is_float = std::is_same<float_type, float>::value;
+  static const bool is_float = std::is_same<float_type, float>::value;
   static const std::string class_name = is_float ? "Float" : "Double";
   if(std::isnan(value))
     return class_name + ".NaN";