sysroot: Add an unload() API

This will be used by gnome-continuous at least to drop the reference
to the fd so that unmounting can proceed.  See
https://git.gnome.org/browse/gnome-continuous/commit/?id=95e9910ea288d302509ca667e0d190dd89377dd5

src/libostree/ostree-sysroot.h

@@ -45,6 +45,8 @@ gboolean ostree_sysroot_load (OstreeSysroot  *self,
                               GCancellable   *cancellable,
                               GError        **error);
 
+void ostree_sysroot_unload (OstreeSysroot  *self);
+
 gboolean ostree_sysroot_ensure_initialized (OstreeSysroot  *self,
                                             GCancellable   *cancellable,
                                             GError        **error);