include: add cached copy of linux/kernel.h

This fixes the following compilation warning when compiling with old
kernel headers.

  CC     expr/target.lo
expr/target.c: In function ‘nft_rule_expr_target_build’:
expr/target.c:127: warning: implicit declaration of function ‘__ALIGN_KERNEL’

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>