kbuild: stop docproc segfaulting when SRCTREE isn't set.

Prevent docproc from segfaulting when SRCTREE isn't set.

Signed-off-by: Rob Landley <rob@landley.net>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>