gsm: Fix make distcheck with parallel make

Change-Id: I0bab4cfbc82d2b0aa7bd07769000ab8e4968a00b
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 152eb60..ed87aca 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -285,7 +285,7 @@
 bitgen_bitgen_test_LDADD = $(LDADD)
 
 gad_gad_test_SOURCES = gad/gad_test.c
-gad_gad_test_LDADD = $(LDADD) $(top_builddir)/src/gsm/gad.o
+gad_gad_test_LDADD = $(LDADD) $(top_builddir)/src/gsm/libosmogsm.la
 
 bsslap_bsslap_test_SOURCES = bsslap/bsslap_test.c
 bsslap_bsslap_test_LDADD = $(LDADD) $(top_builddir)/src/gsm/libosmogsm.la