avcodec/internal: silence -Wempty-body on clang

This silences a -Wempty-body warning on clang 3.7+, e.g
http://fate.ffmpeg.org/log.cgi?time=20150918181527&log=compile&slot=x86_64-darwin-clang-polly-vectorize-stripmine-3.7.

Signed-off-by: Ganesh Ajjanagadde <gajjanagadde@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/internal.h

@@ -57,7 +57,7 @@
 #ifdef TRACE
 #   define ff_tlog(ctx, ...) av_log(ctx, AV_LOG_TRACE, __VA_ARGS__)
 #else
-#   define ff_tlog(ctx, ...) while(0)
+#   define ff_tlog(ctx, ...) while(0) {}
 #endif