kvm: vmx: Fix -Wmissing-prototypes warnings

We get a warning when build kernel W=1:
arch/x86/kvm/vmx/vmx.c:6365:6: warning: no previous prototype for ‘vmx_update_host_rsp’ [-Wmissing-prototypes]
 void vmx_update_host_rsp(struct vcpu_vmx *vmx, unsigned long host_rsp)

Add the missing declaration to fix this.

Signed-off-by: Yi Wang <wang.yi59@zte.com.cn>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>