repo: Add an API to init OstreeSePolicy from commit directly

This is part of `OstreeCommitModifier`, but I'm not using
that in some of the ostree-ext Rust code.

It just makes more sense as a direct policy API, where it should
have been in the first place.  There's already support for
setting a policy object on a commit modifier, so that's all the
old API needs to do now.

src/libostree/libostree-devel.sym

@@ -22,6 +22,11 @@
    - uncomment the include in Makefile-libostree.am
 */
 
+LIBOSTREE_2021.5 {
+global:
+  ostree_sepolicy_new_from_commit;
+} LIBOSTREE_2021.4;
+
 /* Stub section for the stable release *after* this development one; don't
  * edit this other than to update the year.  This is just a copy/paste
  * source.  Replace $LASTSTABLE with the last stable version, and $NEWVERSION