message: move -Wextra-semi to GCC 8.0+
diff: @@ -358,7 +358,6 @@ else()
  -Wno-unused-parameter
  -Wold-style-cast
  -Wimplicit-fallthrough
  - -Wextra-semi
  -fsigned-char
  -Wnon-virtual-dtor
  @@ -381,6 +380,9 @@ else()
  -faligned-new
  -Werror=implicit-fallthrough=2
  >
  + $<$<VERSION_GREATER:$<CXX_COMPILER_VERSION>,8.0>:
  + -Wextra-semi
  + >
  >
  $<$<BOOL:${FLATBUFFERS_CODE_COVERAGE}>:
  