x86/xen: Remove duplicate #include

./arch/x86/xen/enlighten.c: linux/memblock.h is included more than once.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20240322063957.94159-1-jiapeng.chong@linux.alibaba.com

Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=8610