net/intel-ixl-kmod: Add had-linked man page

Summary:

• ixl has two hard-linked man pages: ixl(4) and if_ixl(4), before the port was only installing ixl(4)
• Fix portline warning for USES while here

Test Plan: Compile/sanity tested with poudriere w/portlint

Approved By: sbruno (mentor)

Differential Revision: https://reviews.freebsd.org/D17435