xsettings: Fix build error due to missing gnome-settings-bus.h include

The gsd-xsettings-manager.c code uses gnome_settings_is_wayland() which is declared in gnome-settings-bus.h, add this to the includes.

This fixes the following build-error for me:

../plugins/xsettings/gsd-xsettings-manager.c: In function ‘gsd_xsettings_manager_start’: ../plugins/xsettings/gsd-xsettings-manager.c:1396:13: error: implicit declaration of function ‘gnome_settings_is_wayland’ [-Werror=implicit-function-declaratio] 1396 | if (gnome_settings_is_wayland ()) | ^~~~~~~~~~~~~~~~~~~~~~~~~ cc1: some warnings being treated as errors