Add missing const to last argument of decode_significance_8x8_x86.

Fixes "h264.c:5470: warning: passing argument 4 of 'decode_significance_8x8_x86' discards qualifiers from pointer target type"

Originally committed as revision 11135 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/cabac.h

@@ -738,7 +738,7 @@ static int decode_significance_x86(CABACContext *c, int max_coeff, uint8_t *sign
     return coeff_count;
 }
 
-static int decode_significance_8x8_x86(CABACContext *c, uint8_t *significant_coeff_ctx_base, int *index, uint8_t *sig_off){
+static int decode_significance_8x8_x86(CABACContext *c, uint8_t *significant_coeff_ctx_base, int *index, const uint8_t *sig_off){
     int minusindex= 4-(int)index;
     int coeff_count;
     long last=0;