Fix the build without wayland/kwayland

This fixes the build of plasma-framework on KDE CI. Without this it will
error out as,

dialog.cpp(309): error C2065: 'shellSurface': undeclared identifier
dialog.cpp(310): error C2065: 'shellSurface': undeclared identifier
dialog.cpp(310): error C2227: left of '->setPosition' must point to
class/struct/union/generic type

Signed-off-by: Bhushan Shah <bhush94@gmail.com>