Fix AtkRelationSet leak in xfce_gtk_label_set_a11y_relation

Leak report

Direct leak of 32 byte(s) in 1 object(s) allocated from:
    #0 0x5dc76b9c5c59 in calloc (/usr/asan/bin/thunar+0x498c59) (BuildId: dca53631ecb41a9f)
    #1 0x762f24addb62 in g_malloc0 /usr/src/debug/glib2/build/../glib/glib/gmem.c:133:13
    #2 0x762f25fc17b5 in g_type_create_instance /usr/src/debug/glib2/build/../glib/gobject/gtype.c:1933:17
    #3 0x762f25fa6684 in g_object_new_internal /usr/src/debug/glib2/build/../glib/gobject/gobject.c:2606:24
    #4 0x762f25fa7cfe in g_object_new_internal /usr/src/debug/glib2/build/../glib/gobject/gobject.c:2603:6
    #5 0x762f25fa7cfe in g_object_new_with_properties /usr/src/debug/glib2/build/../glib/gobject/gobject.c:2769:14
    #6 0x762f25fa8d51 in g_object_new /usr/src/debug/glib2/build/../glib/gobject/gobject.c:2415:12
    #7 0x762f264e3f6c in atk_object_init /usr/src/debug/at-spi2-core/build/../at-spi2-core/atk/atkobject.c:659:30
    #8 0x762f25fc180a in g_type_create_instance /usr/src/debug/glib2/build/../glib/gobject/gtype.c:1945:4
    #9 0x762f25fa6684 in g_object_new_internal /usr/src/debug/glib2/build/../glib/gobject/gobject.c:2606:24
    #10 0x762f25fa897a in g_object_new_internal /usr/src/debug/glib2/build/../glib/gobject/gobject.c:2923:29
    #11 0x762f25fa897a in g_object_new_valist /usr/src/debug/glib2/build/../glib/gobject/gobject.c:2945:16
    #12 0x762f25fa8d2f in g_object_new /usr/src/debug/glib2/build/../glib/gobject/gobject.c:2418:12
    #13 0x762f25c09e6a in gtk_widget_real_get_accessible gtk/gtkwidget.c:13566:24
    #14 0x762f265483d4 in xfce_gtk_label_set_a11y_relation libxfce4ui/libxfce4ui/xfce-gtk-extensions.c:1218:44
    #15 0x762f1fb3484a in thunar_apr_desktop_page_init thunar/plugins/thunar-apr/thunar-apr-desktop-page.c:453:3
[...snip...]

Steps to reproduce

• Right-click on a .desktop file in Thunar
• Select Properties to open the properties dialog

Testing

• No issues were detected with LSan, ASan, or Valgrind when testing the patch