kbuild: docbook: fix the include error when executing "make help"

The commit ec3fadd64b99 (kbuild: docbook: use $(obj) and $(src) rather
than specific path) replaces the specific path with $(src). But when
executing "make help", the $(src) is null and then causes an include
error. Fix it by restoring the specific path.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>