Add a const qualifier in a cast, fix the gcc warning:

swscale.c: In function ‘planarCopy’:
swscale.c:2256: warning: cast discards qualifiers from pointer target type

Originally committed as revision 30340 to svn://svn.mplayerhq.hu/mplayer/trunk/libswscale

libswscale/swscale.c

@@ -2253,7 +2253,7 @@ static int planarCopy(SwsContext *c, const uint8_t* src[], int srcStride[], int
 
                 for (i=0; i<height; i++) {
                     for (j=0; j<length; j++)
-                        ((uint16_t*)dstPtr)[j] = bswap_16(((uint16_t*)srcPtr)[j]);
+                        ((uint16_t*)dstPtr)[j] = bswap_16(((const uint16_t*)srcPtr)[j]);
                     srcPtr+= srcStride[plane];
                     dstPtr+= dstStride[plane];
                 }