swscale/rgb2rgb_template: Fix signedness of v in shuffle_bytes_2103_c()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 76043580)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/rgb2rgb_template.c

@@ -322,7 +322,7 @@ static inline void shuffle_bytes_2103_c(const uint8_t *src, uint8_t *dst,
     uint8_t *d       = dst - idx;
 
     for (; idx < 15; idx += 4) {
-        register int v        = *(const uint32_t *)&s[idx], g = v & 0xff00ff00;
+        register unsigned v   = *(const uint32_t *)&s[idx], g = v & 0xff00ff00;
         v                    &= 0xff00ff;
         *(uint32_t *)&d[idx]  = (v >> 16) + g + (v << 16);
     }