smpp: Fix the make distcheck for smpp

DISTCHECK_CONFIGURE_FLAGS="--enable-smpp" make distcheck
diff --git a/openbsc/tests/smpp/Makefile.am b/openbsc/tests/smpp/Makefile.am
index eb25965..b3d4568 100644
--- a/openbsc/tests/smpp/Makefile.am
+++ b/openbsc/tests/smpp/Makefile.am
@@ -7,6 +7,6 @@
 noinst_PROGRAMS = smpp_test
 
 smpp_test_SOURCES = smpp_test.c \
-	$(top_srcdir)/src/libmsc/smpp_utils.c
+	$(top_builddir)/src/libmsc/smpp_utils.c
 smpp_test_LDADD = $(LIBOSMOCORE_LIBS) \
-	$(top_srcdir)/src/libcommon/libcommon.a
+	$(top_builddir)/src/libcommon/libcommon.a