gtphub: add enable_gtphub_test to atlocal.in.

This line was missing, I noticed only now.
diff --git a/openbsc/tests/atlocal.in b/openbsc/tests/atlocal.in
index 3b03bb7..7475f63 100644
--- a/openbsc/tests/atlocal.in
+++ b/openbsc/tests/atlocal.in
@@ -3,3 +3,4 @@
 enable_bsc_test='@osmo_ac_build_bsc@'
 enable_mgcp_transcoding_test='@osmo_ac_mgcp_transcoding@'
 enable_sgsn_test='@found_libgtp@'
+enable_gtphub_test='@found_libgtp@'