core: Make ostree_set_xattrs() private

Nothing external uses it.  We keep ostree_get_xattrs_for_file() public
because it's convenient for external consumers to get xattrs in
exactly the format we desire.

https://bugzilla.gnome.org/show_bug.cgi?id=707733

src/libostree/ostree-core-private.h

@@ -80,6 +80,10 @@ _ostree_set_xattrs_fd (int            fd,
                        GCancellable  *cancellable,
                        GError       **error);
 
+gboolean _ostree_set_xattrs (GFile *f, GVariant *xattrs,
+                             GCancellable *cancellable, GError **error);
+
+
 /* XX + / + checksum-2 + . + extension, but let's just use 256 for a
  * bit of overkill.
  */