kvm: vmx: fix some -Wmissing-prototypes warnings

We get some warnings when building kernel with W=1:
arch/x86/kvm/vmx/vmx.c:426:5: warning: no previous prototype for ‘kvm_fill_hv_flush_list_func’ [-Wmissing-prototypes]
arch/x86/kvm/vmx/nested.c:58:6: warning: no previous prototype for ‘init_vmcs_shadow_fields’ [-Wmissing-prototypes]

Make them static to fix this.

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