KVM: x86: set "mitigate_smt_rsb" storage-class-specifier to static

smatch reports
arch/x86/kvm/x86.c:199:20: warning: symbol
  'mitigate_smt_rsb' was not declared. Should it be static?

This variable is only used in one file so it should be static.

Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20230404010141.1913667-1-trix@redhat.com


Signed-off-by: Sean Christopherson <seanjc@google.com>