net-mgmt/ipv6mon: fix installation

Port uses <bsd.prog.mk>, which desires STRIP=${STRIP}, but we have
started to set STRIP=${STRIP_CMD} in MAKE_ENV a while ago.
Override this bogus setting in MAKE_ARGS to fix installation.

MFH: 2026Q2
See also: D55509
Approved by: portmgr (build fix blanket)