[build] Create a libbsc.a as noinst_LIBRARY, link bsc_hack and tests against it

Link the tests against the libbsc.a to avoid most of the breakage
when creating new symbols.
diff --git a/include/openbsc/Makefile.am b/include/openbsc/Makefile.am
index 7c848b3..93e3c7f 100644
--- a/include/openbsc/Makefile.am
+++ b/include/openbsc/Makefile.am
@@ -2,4 +2,4 @@
 		 gsm_subscriber.h linuxlist.h msgb.h select.h tlv.h gsm_04_11.h \
 		 timer.h misdn.h chan_alloc.h telnet_interface.h paging.h \
 		 subchan_demux.h trau_frame.h e1_input.h trau_mux.h signal.h \
-		 gsm_utils.h
+		 gsm_utils.h ipaccess.h