mgw: Allow receiving lowercase X-Osmo-Ign Callid field

MGCP RFC3435 (https://tools.ietf.org/html/rfc3435) states almost all
text has to be handled in a case-insensitive way, except SDP parts.

Related: OS#4001

Change-Id: Ifc1b3bfe6ff6922df478cea89bbbb291b5fa5706
diff --git a/src/libosmo-mgcp/mgcp_protocol.c b/src/libosmo-mgcp/mgcp_protocol.c
index fa41a8a..7683f4e 100644
--- a/src/libosmo-mgcp/mgcp_protocol.c
+++ b/src/libosmo-mgcp/mgcp_protocol.c
@@ -785,7 +785,7 @@
 		if (!token)
 			break;
 
-		if (!strcmp(token, "C"))
+		if (!strcasecmp(token, "C"))
 			endp->x_osmo_ign |= MGCP_X_OSMO_IGN_CALLID;
 		else
 			LOGPENDP(endp, DLMGCP, LOGL_ERROR, "received unknown X-Osmo-IGN item '%s'\n", token);