Disable -Wenum-compare-switch warning

This is a new warning type in Clang which causes build breakage in
protobuf, so disable it for now.

Change-Id: I8ed2ecff84564021eee907ca87ecdee759ace7be
diff --git a/BUILD.gn b/BUILD.gn
index 3e3ed72..cc6ff53 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -43,7 +43,8 @@
     cflags += [ "-Wno-unused-function" ]
     # Note(rudominer) I added these because there were a lot of warnings.
     cflags += [ "-Wno-sign-compare",
-                "-Wno-missing-field-initializers" ]
+                "-Wno-missing-field-initializers",
+                "-Wno-enum-compare-switch" ]
   }
 }