XfwWorkspaceWayland: fix ext-workspace activate
...to enable compositor workspaces to be switched using the xfce4-panel 'Workspace Switcher' plugin.
Without this fix the plugin would correctly indicate the current workspace, but switching did not work by clicking on the plugin.
Helped-by: Consolatis
Cc: @Tamaranch
Can produce a video of behavior before/after if helpful.