"print-sctp.c" doesn't have to include <net/if.h>, and, if it doesn't do

"print-sctp.c" doesn't have to include <net/if.h>, and, if it doesn't do
so, it doesn't need to provide incomplete definitions of "struct mbuf"
or "struct rtentry", either.  If it *does* include <net/if.h>, it
appears to fail to compile on at least some versions of AIX; remove the
include and the incomplete definitions.