fstree: replace lstat with stat

Fixes mingw cross-compilation. lstat() doesn't exist on win32.

It seems to me that stat() is the right function there, to return
informations about the file it refers to.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20191009102025.10179-7-marcandre.lureau@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>