[OML] Add support for ip.access SET ATTRIBUTE message

Since TS 12.21 implements only SET ATTRIBUTE for some object classes,
ip.access had to extend it to be able to set attributes on arbitrary
objects.  We now introduce a function implementing that message.
diff --git a/openbsc/src/abis_nm.c b/openbsc/src/abis_nm.c
index b5f10bd..e00e569 100755
--- a/openbsc/src/abis_nm.c
+++ b/openbsc/src/abis_nm.c
@@ -2662,3 +2662,12 @@
 {
 	return __simple_cmd(bts, NM_MT_IPACC_RESTART);
 }
+
+int abis_nm_ipaccess_set_attr(struct gsm_bts *bts, u_int8_t obj_class,
+				u_int8_t bts_nr, u_int8_t trx_nr, u_int8_t ts_nr,
+				u_int8_t *attr, u_int8_t attr_len)
+{
+	return abis_nm_ipaccess_msg(bts, NM_MT_IPACC_SET_ATTR,
+				    obj_class, bts_nr, trx_nr, ts_nr,
+				     attr, attr_len);
+}