devel/nextpnr: Update to 0.6

While here, also enable GUI as dependencies are already setup for that.

Changelogs:

0.6: https://github.com/YosysHQ/nextpnr/releases/tag/nextpnr-0.6
0.5: https://github.com/YosysHQ/nextpnr/releases/tag/nextpnr-0.5

PR: 275041
Reviewed by: manu (maintainer)
Approved by: tcberner (mentor)
Differential Revision: https://reviews.freebsd.org/D42553