x11-wm/dwm: update to 6.8

This fixes a regression in getatomprop(): https://git.suckless.org/dwm/commit/a9aa0d8ffbb548b0b1f9f755557aef2482c0f820.html

Threads:

• https://lists.suckless.org/dev/2601/35936.html
• https://lists.suckless.org/hackers/2601/19537.html