Fix GCC when use -Wpedantic

nghttp2_int.h:50:33: error: comma at end of enumerator list [-Werror=pedantic]
Fix GCC when use -Wpedantic

nghttp2_hd.h:115:30: error: comma at end of enumerator list [-Werror=pedantic]
Fix GCC when use -Wpedantic

nghttp2_stream.h:94:43: error: comma at end of enumerator list [-Werror=pedantic]
Fix GCC build with -Werror

nghttp2_session.c:1615:9: error: C++ style comments are not allowed in ISO C90 [-Werror]
Fix GCC build with -Werror

libevent-server.c:691:8: error: C++ style comments are not allowed in ISO C90
Merge branch 'gcc' of https://github.com/alagoutte/nghttp2 into alagoutte-gcc