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

[Benjamin Berg <bberg src gnome org>]

commit a2e1f2848c334ae4965e90ec031629cf7add31af
Author: Hans de Goede <hdegoede redhat com>
Date:   Sat Jul 4 17:43:14 2020 +0200

    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

 plugins/xsettings/gsd-xsettings-manager.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/plugins/xsettings/gsd-xsettings-manager.c b/plugins/xsettings/gsd-xsettings-manager.c
index aff0c60d..7b6fe439 100644
--- a/plugins/xsettings/gsd-xsettings-manager.c
+++ b/plugins/xsettings/gsd-xsettings-manager.c
@@ -41,6 +41,7 @@
 #include "gsd-enums.h"
 #include "gsd-xsettings-manager.h"
 #include "gsd-xsettings-gtk.h"
+#include "gnome-settings-bus.h"
 #include "xsettings-manager.h"
 #include "fc-monitor.h"
 #include "gsd-remote-display-manager.h"