sws: add casts to silence pointer type mismatch warnings

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

libswscale/swscale.c

@@ -992,7 +992,7 @@ int attribute_align_arg sws_scale(struct SwsContext *c,
 
         base = srcStride[0] < 0 ? rgb0_tmp - srcStride[0] * (srcSliceH-1) : rgb0_tmp;
 
-        xyz12Torgb48(c, base, src2[0], srcStride[0]/2, srcSliceH);
+        xyz12Torgb48(c, (uint16_t*)base, (const uint16_t*)src2[0], srcStride[0]/2, srcSliceH);
         src2[0] = base;
     }