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/include/Makefile.am b/openbsc/include/Makefile.am
index a4acc23..4596b6e 100644
--- a/openbsc/include/Makefile.am
+++ b/openbsc/include/Makefile.am
@@ -1,3 +1,3 @@
-SUBDIRS = openbsc sccp
+SUBDIRS = openbsc
 
 noinst_HEADERS = mISDNif.h compat_af_isdn.h