x86/cpu: Provide a declaration for itlb_multihit_kvm_mitigation

Sparse complains rightfully about the missing declaration which has been
placed sloppily into the usage site:

  bugs.c:2223:6: sparse: warning: symbol 'itlb_multihit_kvm_mitigation' was not declared. Should it be static?

Add it to <asm/spec-ctrl.h> where it belongs and remove the one in the KVM code.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20240304005104.787173239@linutronix.de