4xm: make mcdc() src const

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/4xm.c

@@ -281,7 +281,7 @@ static void init_mv(FourXContext *f)
     }
 #endif
 
-static inline void mcdc(uint16_t *dst, uint16_t *src, int log2w,
+static inline void mcdc(uint16_t *dst, const uint16_t *src, int log2w,
                         int h, int stride, int scale, unsigned dc)
 {
     int i;