net/isboot-kmod: fix build on 11-current

Commit r293886 removed rtrequest() from sys/net/route.c in 11-CURRENT.
This update patches isboot.c to call rtrequest_fib() instead. I
have already sent the patch to the isboot maintainer, so hopefully
this is only needed until the next point release.

This allows the port to build on 11-CURRENT as well as 10.x.

PR: 209024
Submitted by: John Nielsen <john@jnielsen.net> (maintainer)