main.c: fix build against gcc-14 (-Walloc-size)

`gcc-14` added a new `-Walloc-size` warning that makes sure that size of
an individual element matches size of a pointed type:

        https://gcc.gnu.org/PR71219

`swaybg` triggers it on `calloc()` calls where member size is used as
`1` (instead of member count):

    ../main.c:492:32: error: allocation of insufficient size '1' for type 'struct swaybg_output_config' with size '48' [-Werror=alloc-size]
      492 |                         config = calloc(sizeof(struct swaybg_output_config), 1);
          |                                ^

0 files changed, 0 insertions, 0 deletions