bsc: Add module for handling BSSAP input

The current implementation is a stub. The code from
bssap.c of on-waves/bsc-master will be migrated into
this new structure
diff --git a/openbsc/src/bsc/Makefile.am b/openbsc/src/bsc/Makefile.am
index 168c721..fe2dff0 100644
--- a/openbsc/src/bsc/Makefile.am
+++ b/openbsc/src/bsc/Makefile.am
@@ -7,7 +7,7 @@
 
 osmo_bsc_SOURCES = osmo_bsc_main.c osmo_bsc_rf.c osmo_bsc_vty.c osmo_bsc_api.c \
 		   osmo_bsc_grace.c osmo_bsc_msc.c osmo_bsc_sccp.c \
-		   osmo_bsc_filter.c \
+		   osmo_bsc_filter.c osmo_bsc_bssap.c \
 		   $(top_srcdir)/src/debug.c $(top_srcdir)/src/bsc_msc.c \
 		   $(top_srcdir)/src/bsc_init.c
 osmo_bsc_LDADD = $(top_builddir)/src/libvty.a \