abis_nm: introduce abis_nm_{osmo,ipa}_magic[]

... used to be oml_{osmo,ipa}_magic in osmo-bts
diff --git a/include/osmocom/gsm/abis_nm.h b/include/osmocom/gsm/abis_nm.h
index bc13ffe..5520f99 100644
--- a/include/osmocom/gsm/abis_nm.h
+++ b/include/osmocom/gsm/abis_nm.h
@@ -10,6 +10,9 @@
 
 /*! \file abis_nm.h */
 
+extern const char abis_nm_ipa_magic[13];
+extern const char abis_nm_osmo_magic[12];
+
 enum abis_nm_msgtype;
 enum gsm_phys_chan_config;