lavu: disable av_uninit for clang

This silence bunch of useless warnings like:

libavformat/mpeg.c:393:37: warning: variable 'dvdaudio_substream_type' is uninitialized when used within its own initialization [-Wuninitialized]
Signed-off-by: Paul B Mahol <onemda@gmail.com>

libavutil/attributes.h

@@ -131,7 +131,7 @@
 #   define av_alias
 #endif
 
-#if defined(__GNUC__) && !defined(__INTEL_COMPILER)
+#if defined(__GNUC__) && !defined(__INTEL_COMPILER) && !defined(__clang__)
 #    define av_uninit(x) x=x
 #else
 #    define av_uninit(x) x