encap4_input: refer to ipip_input only when MROUTING is defined (necessary to make it compile on bsdi4.  don't know implication for others)
encap4_input: refer to ipip_input only when MROUTING is defined
(necessary to make it compile on bsdi4.  don't know implication for
others)

MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame,MarginC/kame