Fixes memory leak:
==995170== 240 (48 direct, 192 indirect) bytes in 1 blocks are definitely lost in loss record 14,428 of 15,641
==995170== at 0x48445EF: calloc (vg_replace_malloc.c:1328)
==995170== by 0x4B211D0: g_malloc0 (gmem.c:155)
==995170== by 0x4A3CDB3: meta_wayland_activation_init (meta-wayland-activation.c:383)
==995170== by 0x4A3FB0C: meta_wayland_compositor_new (meta-wayland.c:636)
==995170== by 0x49C7FA7: meta_context_start (meta-context.c:412)
==995170== by 0x10F065: main (mutter.c:148)
Fixes: ec390b68c5 ("wayland: Implement the xdg-activation protocol")
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2527>