[PATCH] docbook build fix

Fix documentation build with `make O='

Cc: Martin Waitz <tali@admingilde.org>
Cc: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>