gtphub: wrap gtphub_write() for test suite.

Sponsored-by: On-Waves ehi
diff --git a/openbsc/tests/gtphub/Makefile.am b/openbsc/tests/gtphub/Makefile.am
index bcaf4fa..dcb7211 100644
--- a/openbsc/tests/gtphub/Makefile.am
+++ b/openbsc/tests/gtphub/Makefile.am
@@ -13,7 +13,8 @@
 gtphub_test_SOURCES = gtphub_test.c
 gtphub_test_LDFLAGS = \
 	-Wl,--wrap=gtphub_resolve_ggsn_addr \
-	-Wl,--wrap=gtphub_ares_init
+	-Wl,--wrap=gtphub_ares_init \
+	-Wl,--wrap=gtphub_write
 
 gtphub_test_LDADD = \
 	$(top_builddir)/src/gprs/gtphub.o \