Used by libbsc, libmsc as well as osmo-bsc and osmo-bsc_nat.
Moving gsm48_create* to libcommon-cs affects linking of osmo-bsc_nat, resulting in
undefined references to gsm48_extract_mi() and gsm48_paging_extract_mi(); fix
that by placing libfilter.a left of libbsc.a upon linker invocation.
Change-Id: I212c2567b56191022b683674c1c4daf842839946