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/libostree.sym

@@ -390,6 +390,7 @@ LIBOSTREE_2017.4 {
 global:
   ostree_check_version;
   ostree_diff_dirs_with_options;
+  ostree_sepolicy_new_at;
 } LIBOSTREE_2017.3;
 
 /* Stub section for the stable release *after* this development one; don't