backend: fix build against upcoming `gcc-14` (`-Werror=calloc-transposed-args`)

`gcc-14` added a new `-Wcalloc-transposed-args` warning recently. It
detected minor infelicity in `calloc()` API usage in `wlroots`:

    ../backend/libinput/tablet_pad.c: In function 'add_pad_group_from_libinput':
    ../backend/libinput/tablet_pad.c:36:38: error: 'calloc' sizes specified with 'sizeof' in the earlier argument and not in the later argument [-Werror=calloc-transposed-args]
       36 |         group->rings = calloc(sizeof(unsigned int), group->ring_count);
          |                                      ^~~~~~~~
    ../backend/libinput/tablet_pad.c:36:38: note: earlier argument should specify number of elements, later size of each element

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