Bug#1074981: gdl: FTBFS with gcc-14

[Rebecca N. Palmer]

Control: forwarded -1 https://gitlab.gnome.org/GNOME/gdl/-/issues/9
(actually Gentoo reporting the same bug, but they don't have a fix either)

Upstream are considering archiving this project, which suggests but does 
not prove that removing it from Debian might be reasonable if we can't 
fix this bug.

What looks like the actual error is further back than the above log shows:

gdl-dock-item-grip.c: In function 'gdl_dock_item_grip_init':
gdl-dock-item.c: In function 'gdl_dock_item_class_init':
gdl-dock-item.c:358:44: error: passing argument 1 of 
'gdl_dock_object_class_set_is_compound' from incompatible pointer type 
[-Wincompatible-pointer-types]
   358 |     gdl_dock_object_class_set_is_compound (object_class, FALSE);
       |                                            ^~~~~~~~~~~~
       |                                            |
       |                                            GObjectClass * {aka 
struct _GObjectClass *}
In file included from gdl-dock.h:26,
                  from gdl-dock-item.c:38:
../gdl/gdl-dock-object.h:354:74: note: expected 'GdlDockObjectClass *' 
{aka 'struct _GdlDockObjectClass *'} but argument is of type 
'GObjectClass *' {aka 'struct _GObjectClass *'}
   354 | void          gdl_dock_object_class_set_is_compound 
(GdlDockObjectClass *object_class,
       | ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~