ppp: mppe: Add softdep to arc4

The arc4 crypto is mandatory at ppp_mppe probe time, so let's put a
softdep line, so that the corresponding module gets prepared
gracefully.  Without this, a simple inclusion to initrd via dracut
failed due to the missing dependency, for example.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>