libosmo-mgcp-client: extend the mgcp_client for MGW pooling

At the moment the MGCP Client only supports one MGW per application.
Depending on the requirements of the application one MGW might not offer
the performance needed. Lets add support for an MGCP Client pool that is
backward compatible to existing applications.

Change-Id: Icaaba0e470e916eefddfee750b83f5f65291a6b0
Related: SYS#5091
diff --git a/src/libosmo-mgcp-client/Makefile.am b/src/libosmo-mgcp-client/Makefile.am
index 4f17e53..c472ec1 100644
--- a/src/libosmo-mgcp-client/Makefile.am
+++ b/src/libosmo-mgcp-client/Makefile.am
@@ -32,6 +32,7 @@
 	mgcp_client_vty.c \
 	mgcp_client_fsm.c \
 	mgcp_client_endpoint_fsm.c \
+	mgcp_client_pool.c \
 	$(NULL)
 
 libosmo_mgcp_client_la_LDFLAGS = $(AM_LDFLAGS) -version-info $(MGCP_CLIENT_LIBVERSION)