gmain: Remove some redundant casts
gmain: Remove some redundant casts

These were introducing strict aliasing warnings. Remove them (in line
with other uses of `g_once_init_leave()`).

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib,endlessm/glib