[IPCOMP]: Fix reception of incompressible packets

I made a silly typo by entering IPPROTO_IP (== 0) instead of
IPPROTO_IPIP (== 4).  This broke the reception of incompressible
packets.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>