xen/x86: remove duplicated include from enlighten.c

Remove duplicated include.

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch

)

CC: stable@vger.kernel.org
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>