mgcp-client: Fix missing include in mgcp_client_pool.h

struct vty is used as a param but it is never defined.

Change-Id: Ia27bb20a79966cb90e04720462d24a236a168ada
diff --git a/include/osmocom/mgcp_client/mgcp_client_pool.h b/include/osmocom/mgcp_client/mgcp_client_pool.h
index ba31fb0..726eea1 100644
--- a/include/osmocom/mgcp_client/mgcp_client_pool.h
+++ b/include/osmocom/mgcp_client/mgcp_client_pool.h
@@ -2,6 +2,8 @@
 
 #include <stdbool.h>
 
+#include <osmocom/vty/vty.h>
+
 struct mgcp_client;
 struct mgcp_client_pool;
 struct mgcp_client_pool_member;