kernel.bbclass: fix external module building

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.

make -C $kerneldir _mrproper_scripts deleted this file along with
other things so we resurrect it with this patch.

(From OE-Core rev: 2d114fd0cdf7486f18e53f2bf41c4f559e4b4a8d)

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>