[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/tests/channel/Makefile.am b/openbsc/tests/channel/Makefile.am
index 6b7468c..50fadc5 100644
--- a/openbsc/tests/channel/Makefile.am
+++ b/openbsc/tests/channel/Makefile.am
@@ -5,6 +5,7 @@
 
 channel_test_SOURCES = channel_test.c \
 	$(top_srcdir)/src/db.c \
+	$(top_srcdir)/src/gsm_subscriber_base.c \
 	$(top_srcdir)/src/gsm_subscriber.c \
 	$(top_srcdir)/src/debug.c \
 	$(top_srcdir)/src/timer.c \