Add const qualifiers to unmodified function parameters, fixes the warning:

svq1dec.c:584: warning: passing argument 1 of 'svq1_packet_checksum' discards qualifiers from pointer target type

Originally committed as revision 11536 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/svq1dec.c

@@ -540,7 +540,7 @@ static int svq1_decode_delta_block (MpegEncContext *s, GetBitContext *bitbuf,
   return result;
 }
 
-static uint16_t svq1_packet_checksum (uint8_t *data, int length, int value) {
+static uint16_t svq1_packet_checksum (const uint8_t *data, const int length, int value) {
   int i;
 
   for (i=0; i < length; i++) {