kvm: compile process_smi_save_seg_64() only for x86_64

The process_smi_save_seg_64() function called only in the
process_smi_save_state_64() if the CONFIG_X86_64 is set. This
patch adds #ifdef CONFIG_X86_64 around process_smi_save_seg_64()
to prevent following warning message:

arch/x86/kvm/x86.c:5946:13: warning: ‘process_smi_save_seg_64’ defined but not used [-Wunused-function]
 static void process_smi_save_seg_64(struct kvm_vcpu *vcpu, char *buf, int n)
             ^
Signed-off-by: Alexander Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -5955,6 +5955,7 @@ static void process_smi_save_seg_32(struct kvm_vcpu *vcpu, char *buf, int n)
 	put_smstate(u32, buf, offset, process_smi_get_segment_flags(&seg));
 }
 
+#ifdef CONFIG_X86_64
 static void process_smi_save_seg_64(struct kvm_vcpu *vcpu, char *buf, int n)
 {
 	struct kvm_segment seg;
@@ -5970,6 +5971,7 @@ static void process_smi_save_seg_64(struct kvm_vcpu *vcpu, char *buf, int n)
 	put_smstate(u32, buf, offset + 4, seg.limit);
 	put_smstate(u64, buf, offset + 8, seg.base);
 }
+#endif
 
 static void process_smi_save_state_32(struct kvm_vcpu *vcpu, char *buf)
 {