ostree-repo: new public function ostree_repo_list_refs_ext

It accepts a `flags` argument to control its behavior.  Differently
from `ostree_repo_list_refs`, the `refspec_prefix` is not removed from
the results.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

src/libostree/libostree.sym

@@ -315,4 +315,5 @@ local:
 LIBOSTREE_2016.4 {
 global:
         ostree_repo_get_dfd;
+        ostree_repo_list_refs_ext;
 } LIBOSTREE_2016.3;