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/libostree-devel.sym

@@ -21,6 +21,7 @@
 LIBOSTREE_2017.13 {
 global:
   ostree_checksum_file_at;
+  ostree_repo_checkout_at_options_set_devino;
 } LIBOSTREE_2017.12;
 
 /* Stub section for the stable release *after* this development one; don't