x11/walker: Update to 2.8.0

Changelog:

• https://github.com/abenz1267/walker/releases/tag/v2.7.3
• https://github.com/abenz1267/walker/releases/tag/v2.7.4
• https://github.com/abenz1267/walker/releases/tag/v2.7.5
• https://github.com/abenz1267/walker/releases/tag/v2.8.0

Reported by: GitHub (watch releases)