sccp: Use the external libosmo-sccp as sccp implementation

Add --enable-nat and --enable-osmo-bsc to build applications
requiring the Osmo SCCP library to be installed. We are not
using autodiscover as this is out of fashion.
diff --git a/openbsc/src/nat/bsc_nat.c b/openbsc/src/nat/bsc_nat.c
index 538472a..0a37193 100644
--- a/openbsc/src/nat/bsc_nat.c
+++ b/openbsc/src/nat/bsc_nat.c
@@ -1178,6 +1178,7 @@
 	signal(SIGPIPE, SIG_IGN);
 
 	/* recycle timer */
+	sccp_set_log_area(DSCCP);
 	sccp_close.cb = sccp_close_unconfirmed;
 	sccp_close.data = NULL;
 	bsc_schedule_timer(&sccp_close, SCCP_CLOSE_TIME, 0);