[tests] Change the db_test to link to the libopenbsc.a

This should keep the test compiling in the future. It will
link to the libopenbsc.a to get all symbols. We do this in
the other tests too.
diff --git a/tests/db/Makefile.am b/tests/db/Makefile.am
index 5c9c784..3d9722c 100644
--- a/tests/db/Makefile.am
+++ b/tests/db/Makefile.am
@@ -3,6 +3,6 @@
 
 noinst_PROGRAMS = db_test
 
-db_test_SOURCES = db_test.c $(top_srcdir)/src/db.c $(top_srcdir)/src/gsm_subscriber.c
-db_test_LDADD = -ldl -ldbi
+db_test_SOURCES = db_test.c
+db_test_LDADD = $(top_builddir)/src/libbsc.a -ldl -ldbi