retain cc.4.gz man page for Chelsio T6 NICs

This man page was removed in r225583 when cc.4 was renamed to mod_cc.4
With reintroduction of cc.4 "make installworld; make delete-old" was
no longer convergent.

Reported by: Trond Endrestøl
Reviewed by: np, matthew
Approved by: np, matthew (mentor)
Differential Revision: https://reviews.freebsd.org/D8816