Fix Makefile.am for builddir != srcdir
diff --git a/openbsc/tests/debug/Makefile.am b/openbsc/tests/debug/Makefile.am
index 47437b1..5d6b363 100644
--- a/openbsc/tests/debug/Makefile.am
+++ b/openbsc/tests/debug/Makefile.am
@@ -4,4 +4,4 @@
 
 debug_test_SOURCES = debug_test.c
 debug_test_LDADD = 	$(LIBOSMOCORE_LIBS) \
-			$(top_srcdir)/src/libcommon/libcommon.a
+			$(top_builddir)/src/libcommon/libcommon.a