initrd: Include i8042 kernel module

For some reason, between Linux 4.4.19 and 4.4.20, the atkbd and libps2
kernel modules lost their dependency on i8042 in modules.dep, causing
i8042 not to be included in the initrd. This breaks keyboard in the
initrd, in turn breaking LUKS.

This only happens on the 16.03 branch; on 16.09, it appears i8042 is
pulled into the initrd anyway (through some other dependency,
presumably). But let's include it explicitly.

http://hydra.nixos.org/build/40468431