get_bits: add av_unused tag to cache variable

This silences numerous compiler warnings from skip_bits(),
where the cache variable is not used.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/get_bits.h

@@ -127,7 +127,7 @@ for examples see get_bits, show_bits, skip_bits, get_vlc
 
 #   define OPEN_READER(name, gb)                \
     unsigned int name##_index = (gb)->index;    \
-    unsigned int name##_cache = 0
+    unsigned int av_unused name##_cache = 0
 
 #   define CLOSE_READER(name, gb) (gb)->index = name##_index