mgcp_trunk: move enum mgcp_trunk_type to mgcp_trunk.h

The enum mgcp_trunk_type, which is currently located in mgcp_internal.h
makes more sense in mgcp_trunk.h, so lets move it.

Related: OS#2659
Change-Id: I077121503c44fc112a33f1c946f368414e28f841
diff --git a/include/osmocom/mgcp/mgcp_trunk.h b/include/osmocom/mgcp/mgcp_trunk.h
index 78d65e4..2bd55e6 100644
--- a/include/osmocom/mgcp/mgcp_trunk.h
+++ b/include/osmocom/mgcp/mgcp_trunk.h
@@ -1,5 +1,10 @@
 #pragma once
 
+enum mgcp_trunk_type {
+	MGCP_TRUNK_VIRTUAL,
+	MGCP_TRUNK_E1,
+};
+
 struct mgcp_trunk {
 	struct llist_head entry;