tests/smpp: Fix linking order

At least when using system-wide libtalloc, the LIBOSMOCORE_LIBS
must come last, after the static libraries under libcommon.
diff --git a/openbsc/tests/smpp/Makefile.am b/openbsc/tests/smpp/Makefile.am
index b3d4568..06e7af6 100644
--- a/openbsc/tests/smpp/Makefile.am
+++ b/openbsc/tests/smpp/Makefile.am
@@ -8,5 +8,6 @@
 
 smpp_test_SOURCES = smpp_test.c \
 	$(top_builddir)/src/libmsc/smpp_utils.c
-smpp_test_LDADD = $(LIBOSMOCORE_LIBS) \
-	$(top_builddir)/src/libcommon/libcommon.a
+smpp_test_LDADD = \
+	$(top_builddir)/src/libcommon/libcommon.a \
+	$(LIBOSMOCORE_LIBS)