Fix build on macOS due to not having Wayland
Wayland-specific stuff is guarded by #ifdef Q_OS_UNIX
,
which also matches macOS.
Replaced with #ifdef HAVE_WAYLAND
guard.
The option definition is borrowed from KIO.
See: https://invent.kde.org/frameworks/kio/-/blob/c36a98157c7ece4ccaf784b3346460acfac94ccc/CMakeLists.txt#L129