kernel-doc: init kernel version

The kernel-doc script triggers a perl warning when invoked
without KERNELVERSION in the environment, rather make it use
the string "unknown kernel version" instead.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>