Disable -Wunused-function for BoringSSL

This is the only warning produced by this library. It seems like
upstream they have this off, or they compile things that we're not that
use these other functions.

third_party/com_github_grpc_grpc_swift/BUILD.overlay

@@ -31,6 +31,7 @@ cc_library(
         "Sources/BoringSSL/third_party/**/*.h",
     ]),
     hdrs = glob(["Sources/BoringSSL/include/**/*.h"]),
+    copts = ["-Wno-unused-function"],
     includes = ["Sources/BoringSSL/include"],
 )