filter: Move the gsm 04.08 filter to a common place

For customer requirements we want to be able to do
filtering on the BSC as well. The same messages need
to be scanned and the same access-lists will be looked
at. In the future we might even split traffic based
on the IMSI. Begin with moving the code to a new top
level directory and then renaming and removing the
nat dependency.
diff --git a/openbsc/configure.ac b/openbsc/configure.ac
index 00d1b70..394972f 100644
--- a/openbsc/configure.ac
+++ b/openbsc/configure.ac
@@ -179,6 +179,7 @@
     src/libmsc/Makefile
     src/libmgcp/Makefile
     src/libcommon/Makefile
+    src/libfilter/Makefile
     src/osmo-nitb/Makefile
     src/osmo-bsc/Makefile
     src/osmo-bsc_nat/Makefile