um: Remove unused including <linux/version.h>

Fix the following versioncheck warning:
./arch/um/drivers/vector_kern.c: 11 linux/version.h not needed.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Richard Weinberger <richard@nod.at>