repo: Add wrapper function for setting devino cache on checkout opts

I was trying to use this with pygobject for an OCI+ostree project, and pygobject
rejected simply assigning to the field (understandably, since it can't bind the
lifetime together).

Add a wrapper function, which is still unsafe, but hides that unsafety
where most people shouldn't find it.  And if they do...well, sorry,
Rust wasn't invented when ostree was started.

Closes: #1295
Approved by: pwithnall

src/libostree/ostree-repo-commit.c

@@ -3415,6 +3415,8 @@ ostree_repo_commit_modifier_set_sepolicy (OstreeRepoCommitModifier
  *
  * This function will add a reference to @cache without copying - you
  * should avoid further mutation of the cache.
+ *
+ * Since: 2017.13
  */
 void
 ostree_repo_commit_modifier_set_devino_cache (OstreeRepoCommitModifier              *modifier,