filter: Move the method definition to the filter module

Move the filter methods to the filter module. This is
still only usable for the NAT and the _dt/_cr filter
routines need to move back to the bsc_nat in the long
run.
diff --git a/openbsc/tests/bsc-nat/bsc_nat_test.c b/openbsc/tests/bsc-nat/bsc_nat_test.c
index 5b01cc3..105fec9 100644
--- a/openbsc/tests/bsc-nat/bsc_nat_test.c
+++ b/openbsc/tests/bsc-nat/bsc_nat_test.c
@@ -26,6 +26,7 @@
 #include <openbsc/gsm_data.h>
 #include <openbsc/bsc_nat.h>
 #include <openbsc/bsc_nat_sccp.h>
+#include <openbsc/bsc_msg_filter.h>
 #include <openbsc/nat_rewrite_trie.h>
 
 #include <osmocom/core/application.h>