[JFFS2] Fix compr_rubin.c build after include file elimination.

It seems to be silly season lately.

(Oops, test builds are more useful if the file in question is actually
configured on. dwmw2).

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>