constify some function arg pointers

Change-Id: I7a7560fad96719da01f1ee30eea0be0e52c60e99
diff --git a/src/libosmo-mgcp/mgcp_protocol.c b/src/libosmo-mgcp/mgcp_protocol.c
index 3014048..8d6f6f5 100644
--- a/src/libosmo-mgcp/mgcp_protocol.c
+++ b/src/libosmo-mgcp/mgcp_protocol.c
@@ -611,8 +611,8 @@
 		 rtp->force_constant_ssrc ? ", force constant ssrc" : "");
 }
 
-uint32_t mgcp_rtp_packet_duration(struct mgcp_endpoint *endp,
-				  struct mgcp_rtp_end *rtp)
+uint32_t mgcp_rtp_packet_duration(const struct mgcp_endpoint *endp,
+				  const struct mgcp_rtp_end *rtp)
 {
 	int f = 0;