[subscr] Split gsm_subscriber.c in BSC and MSC part

For the BSC part we still assign a gsm_subscriber to lchan but it
might only contain the TMSI of this subscriber.

For the MSC part we will need the HLR/VLR feature of the gsm_subscriber,
specially the lookup's by number...

So if libbsc.a/libmsc.a are compiled in one app and used the
subscribers will be shared, and if only libbsc.a gets used we will
have more empty gsm_subscriber.c..
diff --git a/openbsc/src/Makefile.am b/openbsc/src/Makefile.am
index f5febbc..d90e483 100644
--- a/openbsc/src/Makefile.am
+++ b/openbsc/src/Makefile.am
@@ -7,7 +7,7 @@
 
 libbsc_a_SOURCES = abis_rsl.c abis_nm.c gsm_04_08.c gsm_data.c mncc.c \
 		msgb.c select.c chan_alloc.c timer.c debug.c \
-		gsm_04_11.c subchan_demux.c \
+		gsm_subscriber_base.c gsm_04_11.c subchan_demux.c \
 		trau_frame.c trau_mux.c paging.c e1_config.c e1_input.c tlv_parser.c \
 		input/misdn.c input/ipaccess.c signal.c gsm_utils.c talloc.c talloc_ctx.c \
 		transaction.c rtp_proxy.c bsc_rll.c token_auth.c rrlp.c