filter: Separate SCCP/BSSAP extraction and gsm48 code

For the BSC we will have the gsm48_hdr and don't need to
find data within SCCP. For legacy reasons we need to
initialize con_type, imsi, reject causes early on and
need to do the same in the filter method.
diff --git a/openbsc/src/osmo-bsc_nat/Makefile.am b/openbsc/src/osmo-bsc_nat/Makefile.am
index 7a38430..d96a391 100644
--- a/openbsc/src/osmo-bsc_nat/Makefile.am
+++ b/openbsc/src/osmo-bsc_nat/Makefile.am
@@ -7,7 +7,7 @@
 
 osmo_bsc_nat_SOURCES = bsc_filter.c bsc_mgcp_utils.c bsc_nat.c bsc_nat_utils.c \
 		  bsc_nat_vty.c bsc_sccp.c bsc_ussd.c bsc_nat_ctrl.c \
-		  bsc_nat_rewrite.c bsc_nat_rewrite_trie.c
+		  bsc_nat_rewrite.c bsc_nat_rewrite_trie.c bsc_nat_filter.c
 osmo_bsc_nat_LDADD = \
 		$(top_builddir)/src/libmgcp/libmgcp.a \
 		$(top_builddir)/src/libbsc/libbsc.a \