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

Bug: 56257
Change-Id: I4d739e44eea421cf347243ca91fb1500dd2f88c2
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/github.com/google/cppdap/+/602344
Reviewed-by: Tamir Duberstein <tamird@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index 12dfe13..cbdd56d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -32,7 +32,7 @@
   cflags = [ "-DCPPDAP_JSON_RAPID" ]
 
   # Suppress warnings that Fuchsia build adds
-  cflags += [ "-Wno-extra-semi" ]
+  configs += [ "//build/config:Wno-extra-semi" ]
 
   public_configs = [ ":includes" ]