[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>
diff --git a/BUILD.gn b/BUILD.gn
index ccb816c..8a3dee0 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -15,6 +15,8 @@
   if (is_clang) {
     cflags = [
       "-Wno-conversion",
+      "-Wno-implicit-fallthrough",
+
       # libpng checks that the width is not greater than PNG_SIZE_MAX.
       # On platforms where size_t is 64-bits, this comparison will always
       # be false.