Disable implicit-fallthrough warning.

Change-Id: Ib94f521aa0392985b4f03f5ea57a06554ea6acd6
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/curl/+/446781
Fuchsia-Auto-Submit: Alexandre Zani <azani@google.com>
Reviewed-by: Zach Bush <zmbush@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index aeaf8ed..1dd2bc3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -330,6 +330,7 @@
   cflags = [
     "-Wno-string-plus-int",
     "-Wno-conversion",
+    "-Wno-implicit-fallthrough",
   ]
 }