abis: Get the structs and values from libosmocore

Remove the forward declarations for functions and structs
that have been moved to libosmocore.
diff --git a/openbsc/src/libbsc/abis_nm_vty.c b/openbsc/src/libbsc/abis_nm_vty.c
index 3a16b2b..bcbc92d 100644
--- a/openbsc/src/libbsc/abis_nm_vty.c
+++ b/openbsc/src/libbsc/abis_nm_vty.c
@@ -26,6 +26,8 @@
 
 #include <arpa/inet.h>
 
+#include <osmocom/gsm/abis_nm.h>
+
 #include <openbsc/gsm_data.h>
 #include <osmocom/core/msgb.h>
 #include <osmocom/gsm/tlv.h>