[C++] Fix clang -Wnewline-eof warning (#7711)

* Fix clang -Wnewline-eof warning

* Enable -Wnewline-eof warning

Co-authored-by: Derek Bailey <derekbailey@google.com>

tests/monster_test.h

@@ -35,4 +35,4 @@ void UnPackTo(const uint8_t *flatbuf);
 }  // namespace tests
 }  // namespace flatbuffers
 
-#endif
+#endif