avformat/mpjpegdec: Fix "libavformat/mpjpegdec.c:269:9: warning: passing...

avformat/mpjpegdec: Fix "libavformat/mpjpegdec.c:269:9: warning: passing argument 3 of av_stristart from incompatible pointer type"
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mpjpegdec.c

@@ -250,8 +250,8 @@ static int parse_multipart_header(AVIOContext *pb,
 static char* mpjpeg_get_boundary(AVIOContext* pb)
 {
     uint8_t *mime_type = NULL;
-    uint8_t *start;
-    uint8_t *end;
+    const char *start;
+    const char *end;
     uint8_t *res = NULL;
     int     len;