lib/repo-finder: Add OstreeRepoFinderOverride

This is another OstreeRepoFinder implementation; it returns results from
a given set of URIs. It’s designed to be used for implementing user
overrides to other repo-finders, or for implementing unit tests.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

Closes: #1281
Approved by: mwleeds

src/libostree/ostree.h

@@ -40,6 +40,7 @@
 #include <ostree-repo-finder-avahi.h>
 #include <ostree-repo-finder-config.h>
 #include <ostree-repo-finder-mount.h>
+#include <ostree-repo-finder-override.h>
 #endif /* OSTREE_ENABLE_EXPERIMENTAL_API */
 
 #include <ostree-autocleanups.h>