mscsplit: talloc_ctx_init(): decouple from global tall_bsc_ctx

Decouple the talloc context allocations from global tall_bsc_ctx pointer.

It appears that talloc_ctx_init() was intended for general use, since it is
located in libcommon. It is currently used only by osmo-nitb; but the upcoming
osmo-cscn will use it as well.

Instead of defining in osmo-nitb main file, add definition in gsm_data.h.

Change-Id: I168106599b788f586be0ff0af4699b9746c1b103
diff --git a/openbsc/src/osmo-nitb/bsc_hack.c b/openbsc/src/osmo-nitb/bsc_hack.c
index 976aefa..67efe4f 100644
--- a/openbsc/src/osmo-nitb/bsc_hack.c
+++ b/openbsc/src/osmo-nitb/bsc_hack.c
@@ -243,8 +243,6 @@
 	osmo_timer_schedule(&bsc_gsmnet->subscr_expire_timer, EXPIRE_INTERVAL);
 }
 
-void talloc_ctx_init(void);
-
 extern int bsc_vty_go_parent(struct vty *vty);
 
 static struct vty_app_info vty_info = {
@@ -261,7 +259,7 @@
 	vty_info.copyright = openbsc_copyright;
 
 	tall_bsc_ctx = talloc_named_const(NULL, 1, "openbsc");
-	talloc_ctx_init();
+	talloc_ctx_init(tall_bsc_ctx);
 	on_dso_load_token();
 	on_dso_load_rrlp();
 	on_dso_load_ho_dec();