Introduce support for libosmo-mgcp-client MGW pooling

Large RAN installations may benefit from distributing the RTP voice
stream load over multiple media gateways.

libosmo-mgcp-client supports MGW pooling since version 1.8.0 (more than
one year ago). OsmoBSC has already been making use of it since then (see
osmo-bsc.git 8d22e6870637ed6d392a8a77aeaebc51b23a8a50); lets use this
feature in osmo-msc too.

This commit is also part of a series of patches cleaning up
libosmo-mgcp-client and slowly getting rid of the old non-mgw-pooled VTY
configuration, in order to keep only 1 way to configure
libosmo-mgcp-client through VTY.

Related: SYS#5091
Related: SYS#5987
Change-Id: I7670ba56fe989706579224a364595fdd4b4708ff
diff --git a/include/osmocom/msc/vty.h b/include/osmocom/msc/vty.h
index 1e13846..11d9ed1 100644
--- a/include/osmocom/msc/vty.h
+++ b/include/osmocom/msc/vty.h
@@ -17,6 +17,7 @@
 
 enum bsc_vty_node {
 	GSMNET_NODE = _LAST_OSMOVTY_NODE + 1,
+	MGW_NODE,
 	SUBSCR_NODE,
 	MSC_NODE,
 	MNCC_INT_NODE,