sepolicy: Add ostree_sepolicy_new_at()

I'm porting other code away from `GFile`, and while we don't use this
internally, it will let us do so at a later date. I'm averse to changing the
code right now as we don't have good CI coverage of this.

Closes: #746
Approved by: jlebon

src/libostree/ostree-sepolicy.h

@@ -38,6 +38,12 @@ OstreeSePolicy* ostree_sepolicy_new (GFile         *path,
                                      GCancellable  *cancellable,
                                      GError       **error);
 
+_OSTREE_PUBLIC
+OstreeSePolicy* ostree_sepolicy_new_at (int            rootfs_dfd,
+                                        GCancellable  *cancellable,
+                                        GError       **error);
+
+
 _OSTREE_PUBLIC
 GFile * ostree_sepolicy_get_path (OstreeSePolicy  *self);