repo: Add ostree_repo_get_dfd()

I plan to use this in rpm-ostree at least for two reasons:
 - To find the mtime on the repo
 - To use the tmp/ directory to stage content (but we should eventually
   add a better API)

src/libostree/libostree.sym

@@ -311,3 +311,8 @@ global:
 local:
        *;
 };
+
+LIBOSTREE_2016.4 {
+global:
+        ostree_repo_get_dfd;
+} LIBOSTREE_2016.3;