[cpp] Suppress -Wextra-semi

Bug: 56257
Change-Id: Id02f0e3f5c2fa5d59851b884a3bc70d3f9ef1f59
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/flatbuffers/+/600644
Reviewed-by: Tamir Duberstein <tamird@google.com>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index e7db5c7..4714614 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -8,6 +8,7 @@
   cflags = [
     "-Wno-conversion",
     "-Wno-implicit-fallthrough",
+    "-Wno-extra-semi",
   ]
 }