OML: add external alerts

Add special cause for alerts produced by external processes.

Change-Id: Idd7ee085321f8172c72ecfdba320186049f4d988
Related: OS#1615
diff --git a/src/gsm/abis_nm.c b/src/gsm/abis_nm.c
index 9839d29..10aae9a 100644
--- a/src/gsm/abis_nm.c
+++ b/src/gsm/abis_nm.c
@@ -161,6 +161,7 @@
 	{ OSMO_EVT_MAJ_NET_CONGEST,	"Network congestion" },
 	{ OSMO_EVT_MIN_PAG_TAB_FULL,	"Paging table full" },
 	{ OSMO_EVT_WARN_SW_WARN,	"Software warning" },
+	{ OSMO_EVT_EXT_ALARM,		"External alarm" },
 	{ 0, NULL }
 };