background: Fix compile warning about object type

[8/15] Compiling C object 'panels/background/panels@background@@background-chooser@sta/cc-background-xml.c.o'. In file included from /usr/include/glib-2.0/gobject/gbinding.h:29:0, from /usr/include/glib-2.0/glib-object.h:23, from /usr/include/glib-2.0/gio/gioenums.h:28, from /usr/include/glib-2.0/gio/giotypes.h:28, from /usr/include/glib-2.0/gio/gio.h:26, from ../panels/background/cc-background-xml.c:22: ../panels/background/cc-background-xml.c: In function ‘cc_background_xml_load_xml_internal’: /usr/include/glib-2.0/gobject/gobject.h:512:32: warning: passing argument 2 of ‘emit_added_in_idle’ from incompatible pointer type [-Wincompatible-pointer-types] #define g_object_ref(Obj) ((typeof(Obj)) (g_object_ref) (Obj)) ^ ../panels/background/cc-background-xml.c:310:34: note: in expansion of macro ‘g_object_ref’ emit_added_in_idle (xml, g_object_ref (item)); ^~~~~~~~~~~~ ../panels/background/cc-background-xml.c:138:1: note: expected ‘GObject * {aka struct _GObject *}’ but argument is of type ‘CcBackgroundItem * {aka struct _CcBackgroundItem *}’ emit_added_in_idle (CcBackgroundXml *xml, ^~~~~~~~~~~~~~~~~~