show: apply const to right part of pointer

show: apply const to right part of pointer

Without this -Wcast-qual complains:

show.c:30:43: warning: cast from 'const void *' to 'const void **' drops const qualifier [-Wcast-qual]
        const struct wgpeer *a = *(const void **)first, *b = *(const void **)second;
                                                 ^
show.c:30:71: warning: cast from 'const void *' to 'const void **' drops const qualifier [-Wcast-qual]
        const struct wgpeer *a = *(const void **)first, *b = *(const void **)second;

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>