[OML] Allow each BTS model to specify the OML callback

This allows us to add BTS models that do not use the standard TS 12.21 OML
diff --git a/openbsc/src/bts_unknown.c b/openbsc/src/bts_unknown.c
index 2957ce0..f954599 100644
--- a/openbsc/src/bts_unknown.c
+++ b/openbsc/src/bts_unknown.c
@@ -27,6 +27,8 @@
 
 static struct gsm_bts_model model_unknown = {
 	.type = GSM_BTS_TYPE_UNKNOWN,
+	.name = "unknown",
+	.oml_rcvmsg = &abis_nm_rcvmsg,
 	.nm_att_tlvdef = {
 		.def = {
 		},