Prevent tech loss removing self root_req techs.

See bug #22161.

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25289 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Prevent tech loss of tech which is root_req for some other known tech.

Reported by Matthias Pfafferodt (syntron@gna).

See gna bug #19176.

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@25290 a0f10bec-cc02-0410-94fc-a9cfff90b4cd