[cpp] Suppress -Wextra-semi in third party code

Bug: 56257
Change-Id: Icbe8431cfee0b55e65d9c7ce9a142bf4d8123cfa
diff --git a/BUILD.gn b/BUILD.gn
index 6a35b0d..10acaf7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -42,6 +42,7 @@
   if (is_clang) {
     # These are all needed as of https://github.com/protocolbuffers/protobuf/commit/66d7491.
     cflags += [
+      "-Wno-extra-semi",
       "-Wno-float-conversion",
       "-Wno-implicit-float-conversion",
       "-Wno-implicit-int-conversion",