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/bsc/osmo_bsc_main.c b/openbsc/src/bsc/osmo_bsc_main.c
index b1c5001..a678227 100644
--- a/openbsc/src/bsc/osmo_bsc_main.c
+++ b/openbsc/src/bsc/osmo_bsc_main.c
@@ -26,6 +26,8 @@
 #include <osmocom/vty/command.h>
 #include <osmocore/talloc.h>
 
+#include <sccp/sccp.h>
+
 #define _GNU_SOURCE
 #include <getopt.h>
 
@@ -158,6 +160,9 @@
 	/* seed the PRNG */
 	srand(time(NULL));
 
+	/* initialize SCCP */
+	sccp_set_log_area(DSCCP);
+
 
 	rc = bsc_bootstrap_network(NULL, config_file);
 	if (rc < 0) {