Implicit declaration of gdk_wayland_display_query_registry when GTK+ is built without Wayland support

When GTK+ is built with Wayland support disabled (-Dwayland_backend=false), xfce4-panel fails to build with the following warning/error (depending on how the compiler treats implicit declarations; building with --disable-wayland-client --disable-gtk-layer-shell):

panel-application.c: In function ‘panel_application_init’:
panel-application.c:249:13: warning: implicit declaration of function ‘gdk_wayland_display_query_registry’ [-Wimplicit-function-declaration]
  249 |       if (! gdk_wayland_display_query_registry (display, "zwlr_foreign_toplevel_manager_v1"))
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
panel-application.c:249:13: warning: nested extern declaration of ‘gdk_wayland_display_query_registry’ [-Wnested-externs]

Original bug report: https://bugs.gentoo.org/903802