srtp: Include rtpdec.h for RTP_MAX_PACKET_LENGTH

This was missed from 350ad50b - this fixes building the fate-srtp
test.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/srtp.c

@@ -25,6 +25,7 @@
 #include "libavutil/intreadwrite.h"
 #include "libavutil/log.h"
 #include "rtp.h"
+#include "rtpdec.h"
 #include "srtp.h"
 
 void ff_srtp_free(struct SRTPContext *s)