games/chessx: Update 1.6.8 => 1.6.10

Changelog:
https://github.com/Isarhamster/chessx/blob/master/ChangeLog.md

Port changes:

• Mark devel/qt5-concurrent as build dependency explicitly in USE_QT.
• Strip installed binary as it's advised by ports QA.

PR: 295235
Reported by: Alexey Vyskubov <alexey@pentode.fi> (maintainer)
Approved by: osa, vvd (Mentors, implicit)
MFH: 2026Q2

(cherry picked from commit 1314603f671782f01640b3a0599813f08ef75ff9)