keymap: don't use darray in xkb_mod_set

keymap: don't use darray in xkb_mod_set

Instead just statically allocate the mods array (of size MAX_MOD_SIZE =
32). The limit is not going anywhere, and static allocations are nicer
(nicer code, no OOM, etc.). It's also small and dense enough.

Signed-off-by: Ran Benita <ran234@gmail.com>