mgw: Allow receiving lowercase MGCP header keyword

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: I7d1e55faddafa3c3093d38513d4a434ecf5ea5bd
diff --git a/src/libosmo-mgcp/mgcp_msg.c b/src/libosmo-mgcp/mgcp_msg.c
index 44589d1..3e95ed1 100644
--- a/src/libosmo-mgcp/mgcp_msg.c
+++ b/src/libosmo-mgcp/mgcp_msg.c
@@ -325,7 +325,7 @@
 			}
 			break;
 		case 2:
-			if (strcmp("MGCP", elem)) {
+			if (strcasecmp("MGCP", elem)) {
 				LOGP(DLMGCP, LOGL_ERROR,
 				     "MGCP header parsing error\n");
 				return -510;