[protobuf] Supress deprecated-declaration warning on Mac

Mac has deprecated sprintf, combined with -Werror this makes
the protoc build fail.

BUG=https://github.com/dart-lang/sdk/issues/52407

Change-Id: I7f376bb91fb9d7ebc61c85e65187858e81189c1a

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index d6bc4ac..5c1ec9d 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -69,6 +69,7 @@
       "-Wno-sign-compare",
       "-Wno-unused-function",
       "-Wno-unused-private-field",
+      "-Wno-deprecated-declarations", # Mac deprecated sprintf
     ]
   }
 }