x86: kvm: hide the unused 'cpu' variable

The local variable was newly introduced but is only accessed in one
place on x86_64, but not on 32-bit:

arch/x86/kvm/vmx.c: In function 'vmx_save_host_state':
arch/x86/kvm/vmx.c:2175:6: error: unused variable 'cpu' [-Werror=unused-variable]

This puts it into another #ifdef.

Fixes: 35060ed6a1ff ("x86/kvm/vmx: avoid expensive rdmsr for MSR_GS_BASE")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -2351,7 +2351,9 @@ static unsigned long segment_base(u16 selector)
 static void vmx_save_host_state(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
+#ifdef CONFIG_X86_64
 	int cpu = raw_smp_processor_id();
+#endif
 	int i;
 
 	if (vmx->host_state.loaded)