net/intel-ixl-kmod: Remove -Werror from CFLAGS

-Werror in CFLAGS may cause overreaction if a custom compiler or
compilation options are used. To avoid this - WERROR is disabled
in kmod.mk

Compile and sanity checked intel-{ixl,ix,em}-kmod using
poudriere+portlint

Approved By: sbruno (mentor), koobs,

franco_opnsense.org (intel-em-kmod maintainer)

Differential Revision: https://reviews.freebsd.org/D17533