[CMake] Forward OPENSSL_ROOT_DIR to builtin xrootd

It was tested that this works by building openssl on Arch Linux in a
random directory and setting this as the `OPENSSL_ROOT_DIR`.

Then, I checked in the cache for the xrootd builtin external project if
openssl was found in the desired location:
```
builtins/xrootd/BUILTIN_XROOTD-prefix/src/BUILTIN_XROOTD-build/CMakeCache.txt
```

This was the case indeed.

builtins/xrootd/CMakeLists.txt

@@ -38,6 +38,7 @@ ExternalProject_Add(
                -DENABLE_CEPH=OFF
                -DXRDCL_LIB_ONLY=ON
                -DCMAKE_INSTALL_RPATH:STRING=${XROOTD_PREFIX}/lib
+               -DOPENSSL_ROOT_DIR=${OPENSSL_ROOT_DIR}
     INSTALL_COMMAND ${CMAKE_COMMAND} --build . --target install
     LOG_DOWNLOAD 1 LOG_CONFIGURE 1 LOG_BUILD 1 LOG_INSTALL 1 LOG_OUTPUT_ON_FAILURE 1
     BUILD_BYPRODUCTS ${XROOTD_CLIENT_LIBRARIES} ${XROOTD_UTILS_LIBRARIES}