sysutils/msktutil: fix breakage by using OPSYS
Despite passing poudriere testport and bulk and building and working,
this wasn't right. make -V PORTVERSION fails. It does not build.
Instead, thanks to jrm@, this hint from Mk/Uses/gssapi.mk:93 helps
It builds on 15.1 and 14.4, passses portlint, etc.
PR: 292735
(cherry picked from commit R11:7c4426d408ba)