sysutils/rdate: Unbreak DragonFly after previous commit

OSVERSION was used without OPSYS. This fix really should be
"if dragonfly or (freebsd osversion < X)" but currently OSVERSION
is valid as long as OPSYS <> DragonFly so this change will work and
is simpler than the "real" fix. Ideally all of this should be replaced
with a feature detection mechanism.