[config] Suppress implicit fallthrough

Change-Id: I6e422e3c5c4c2af55b6e5e42395b1ae757d64428
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/libpng/+/527271
Reviewed-by: Shai Barack <shayba@google.com>
1 file changed