x86/kvm: Make steal_time visible

This per cpu variable is accessed from assembler code, so it needs
to be visible for LTO.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: pbonzini@redhat.com
Link: https://lkml.kernel.org/r/20190330004743.29541-8-andi@firstfloor.org