[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/json_test.cpp

@@ -171,4 +171,4 @@ void JsonUnsortedArrayTest() {
 }
 
 }  // namespace tests
-}  // namespace flatbuffers
+}  // namespace flatbuffers