gtphub: wrap gtphub_write() for test suite.

Sponsored-by: On-Waves ehi
diff --git a/openbsc/src/gprs/Makefile.am b/openbsc/src/gprs/Makefile.am
index 634948d..295fb86 100644
--- a/openbsc/src/gprs/Makefile.am
+++ b/openbsc/src/gprs/Makefile.am
@@ -34,8 +34,8 @@
 			-lgtp $(OSMO_LIBS) $(LIBOSMOABIS_LIBS) $(LIBCARES_LIBS) \
 			$(LIBCRYPTO_LIBS) -lrt
 
-osmo_gtphub_SOURCES =	gtphub_main.c gtphub.c gtphub_ares.c gtphub_vty.c \
-			sgsn_ares.c gprs_utils.c
+osmo_gtphub_SOURCES =	gtphub_main.c gtphub.c gtphub_sock.c gtphub_ares.c \
+			gtphub_vty.c sgsn_ares.c gprs_utils.c
 osmo_gtphub_LDADD = 	\
 			$(top_builddir)/src/libcommon/libcommon.a \
 			-lgtp $(LIBOSMOCORE_LIBS) $(LIBOSMOGSM_LIBS) $(LIBOSMOVTY_LIBS) \