lavf: Constify the probe function argument.

Reviewed-by: Lauri Kasanen
Reviewed-by: Tomas Härdin

libavformat/ilbc.c

@@ -59,7 +59,7 @@ static int ilbc_write_packet(AVFormatContext *s, AVPacket *pkt)
     return 0;
 }
 
-static int ilbc_probe(AVProbeData *p)
+static int ilbc_probe(const AVProbeData *p)
 {
     // Only check for "#!iLBC" which matches both formats
     if (!memcmp(p->buf, mode20_header, 6))