abis_nm.c: Fix implicit enumeration type conversion

Fixes following warning message:
abis_nm.c:101:2: warning: implicit conversion from enumeration type 'enum abis_nm_msgtype_bs11' to different enumeration type 'enum abis_nm_msgtype' [-Wenum-conversion]

Change-Id: Icb0ed423febf1ceb6626e256d32011b4118fc527
diff --git a/src/gsm/abis_nm.c b/src/gsm/abis_nm.c
index c59ebb7..2ad1945 100644
--- a/src/gsm/abis_nm.c
+++ b/src/gsm/abis_nm.c
@@ -98,10 +98,10 @@
 	NM_MT_CHG_HW_CONF_NACK,
 	NM_MT_GET_ATTR_NACK,
 	NM_MT_SET_ALARM_THRES_NACK,
-	NM_MT_BS11_BEGIN_DB_TX_NACK,
-	NM_MT_BS11_END_DB_TX_NACK,
-	NM_MT_BS11_CREATE_OBJ_NACK,
-	NM_MT_BS11_DELETE_OBJ_NACK,
+	(enum abis_nm_msgtype) NM_MT_BS11_BEGIN_DB_TX_NACK,
+	(enum abis_nm_msgtype) NM_MT_BS11_END_DB_TX_NACK,
+	(enum abis_nm_msgtype) NM_MT_BS11_CREATE_OBJ_NACK,
+	(enum abis_nm_msgtype) NM_MT_BS11_DELETE_OBJ_NACK,
 };
 
 static const struct value_string nack_names[] = {