clang/win: Make libvpx build with -Wunused-function.

BUG=chromium:505316

Change-Id: I1c951eb99fd0487e1bc70ebb2852b981fd69ed8a
diff --git a/vpx/vpx_codec.h b/vpx/vpx_codec.h
index b94e173..b6037bb 100644
--- a/vpx/vpx_codec.h
+++ b/vpx/vpx_codec.h
@@ -69,7 +69,7 @@
 
   /*!\brief Decorator indicating a function is potentially unused */
 #ifdef UNUSED
-#elif __GNUC__
+#elif defined(__GNUC__) || defined(__clang__)
 #define UNUSED __attribute__ ((unused))
 #else
 #define UNUSED