Fix compilation with no libc-ares present on the system

This build failure was introduced with the OAP and gtphub changes.
diff --git a/openbsc/tests/Makefile.am b/openbsc/tests/Makefile.am
index 995660a..04b8e34 100644
--- a/openbsc/tests/Makefile.am
+++ b/openbsc/tests/Makefile.am
@@ -13,7 +13,10 @@
 endif
 
 if HAVE_LIBGTP
-SUBDIRS += sgsn oap gtphub
+SUBDIRS += gtphub
+if HAVE_LIBCARES
+SUBDIRS += sgsn oap
+endif
 endif