kernel.bbclass: fix external module building

Without that fix we have the following while compiling compat-wireless.
  include/linux/mmzone.h:18:30: fatal error: generated/bounds.h: No such file or directory

Note that the compat-wireless recipe will be added in another commit.

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>