x11/xfce4-whiskermenu-plugin: rename GTK_LAYER_SHELL to WAYLAND

Allow X11-only users to globally disable Wayland dependencies via
OPTIONS_UNSET+=WAYLAND in make.conf(5). Besides, gtk-layer-shell will
fail to build unless gtk3 is built with WAYLAND enabled.

PR: 281846
Approved by: madpilot