[otbn,dv] Change otbn_scoreboard::on_alert to not be protected
[otbn,dv] Change otbn_scoreboard::on_alert to not be protected

This now matches the base class (and xcelium spits out a warning if it
doesn't)

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan