gdk: Plug leak in gdk_display_pointer_is_grabbed

gdk: Plug leak in gdk_display_pointer_is_grabbed

gdk_x11_device_manager_core_list_devices returns a new allocated
list, which has to be freed.

valgrind output:

==18686== 160,176 (80,088 direct, 80,088 indirect) bytes in 3,337 blocks are definitely lost in loss record 25,347 of 25,378
==18686==    at 0x4C256DD: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==18686==    by 0x6CD7752: g_malloc (in /lib64/libglib-2.0.so.0.3000.0)
==18686==    by 0x6CEE2B6: g_slice_alloc (in /lib64/libglib-2.0.so.0.3000.0)
==18686==    by 0x6CCB37D: g_list_prepend (in /lib64/libglib-2.0.so.0.3000.0)
==18686==    by 0x654CADA: gdk_x11_device_manager_core_list_devices (gdkdevicemanager-core-x11.c:836)
==18686==    by 0x6531489: gdk_display_pointer_is_grabbed (gdkdisplay.c:1270)
==18686==    by 0x5162E1E: filter_func (ui.c:140)
==18686==    by 0x6558B50: gdk_event_apply_filters (gdkeventsource.c:83)
==18686==    by 0x6558CB3: _gdk_x11_display_queue_events (gdkeventsource.c:197)
==18686==    by 0x6530680: gdk_display_get_event (gdkdisplay.c:311)
==18686==    by 0x65589F1: gdk_event_source_dispatch (gdkeventsource.c:356)
==18686==    by 0x6CD0A0E: g_main_context_dispatch (in /lib64/libglib-2.0.so.0.3000.0)

https://bugzilla.gnome.org/show_bug.cgi?id=660676