lavu/internal: define av_restrict if it has not been defined by config.h

This can happen if a application doesnt use ffmpegs configure
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavutil/internal.h

@@ -53,6 +53,10 @@ struct AVDictionary {
 #endif
 #endif
 
+#ifndef av_restrict
+#define av_restrict restrict
+#endif
+
 #ifndef INT16_MIN
 #define INT16_MIN       (-0x7fff - 1)
 #endif