x86/umip: Make umip_insns static

Fix the following sparse warning:
  arch/x86/kernel/umip.c:84:12: warning: symbol 'umip_insns' was not declared.
  Should it be static?

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Link: https://lkml.kernel.org/r/20200413082213.22934-1-yanaijie@huawei.com