hnbgw: Avoid re-defining same global variable twice

This global variable is already defined in iu_client.c provided by
libosmo-ranap (and used by libasn1c).
Fixes ASan warning at starutp.

Change-Id: I7dbae83ce0dc4ac25dc10879e223280414f7b561

src/hnbgw.c

@@ -72,7 +72,6 @@ static const char * const osmo_hnbgw_copyright =
 	"There is NO WARRANTY, to the extent permitted by law.\r\n";
 
 static void *tall_hnb_ctx;
-void *talloc_asn1_ctx;
 
 static struct hnb_gw *g_hnb_gw;