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/src/osmo-bsc_nat/bsc_nat_utils.c b/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
index 70ad577..e658569 100644
--- a/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
+++ b/openbsc/src/osmo-bsc_nat/bsc_nat_utils.c
@@ -23,6 +23,7 @@
 
 #include <openbsc/bsc_nat.h>
 #include <openbsc/bsc_nat_sccp.h>
+#include <openbsc/bsc_msg_filter.h>
 #include <openbsc/bsc_msc.h>
 #include <openbsc/gsm_data.h>
 #include <openbsc/debug.h>