set -Wno-implicit-fallthrough on GCC too

Bug: skia:7643

Change-Id: Ieeca5414f51c7c113c54b47578ec840fc7d684e0
Reviewed-on: https://skia-review.googlesource.com/109520
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
1 file changed