kernel/kexec_file.c: remove some duplicated includes

kernel/kexec_file.c: remove some duplicated includes

We include kexec.h and slab.h twice in kexec_file.c. It's unnecessary.
hence just remove them.

Link: http://lkml.kernel.org/r/1537498098-19171-1-git-send-email-zhongjiang@huawei.com
Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Reviewed-by: Bhupesh Sharma <bhsharma@redhat.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Baoquan He <bhe@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>