lib/fetcher: Add cleanup function for OstreeFetcher
lib/fetcher: Add cleanup function for OstreeFetcher

This is only used internally (the header is not public), so it doesn’t
have to go in ostree-autocleanups.h. It will be used in some following
commits.

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

Closes: #911
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree