[C++] Fix clang -Wnewline-eof warning

[mogemimi]

Building a flatc-generated C++ header with Clang 14 and -Wnewline-eof option produces the following warnings generated by the inclusion of several header files <include/flatbuffers/*.h>:

flatbuffers/include/flatbuffers/allocator.h(68,36): error : no newline at end of file [-Werror,-Wnewline-eof]
flatbuffers/include/flatbuffers/buffer_ref.h(53,37): error : no newline at end of file [-Werror,-Wnewline-eof]
flatbuffers/include/flatbuffers/default_allocator.h(64,44): error : no newline at end of file [-Werror,-Wnewline-eof]
flatbuffers/include/flatbuffers/string.h(64,33): error : no newline at end of file [-Werror,-Wnewline-eof]
flatbuffers/include/flatbuffers/struct.h(53,33): error : no newline at end of file [-Werror,-Wnewline-eof]

This PR adds a newline at eof to avoid the warnings.

[dbaileychess]

Thanks, can you add this flag to the list here as well? https://github.com/google/flatbuffers/blob/master/CMakeLists.txt#L406

[mogemimi]

@dbaileychess
Thanks for the review! I have added its flag -Wnewline-eof to the CMakeLists.txt. Also, when building flatbuffers with CMake and the flag, there was several similar warnings in other files, which were additionally fixed.

This pull request is rebased to the latest master branch.

[dbaileychess]

Thanks!