x11/libXfixes: update to 6.0.1

Remove GNU_CONFIGURE_MANPREFIX

Changes: https://gitlab.freedesktop.org/xorg/lib/libxfixes/-/compare/libXfixes-6.0.0...libXfixes-6.0.1
Reviewed by: diizzy, jbeich
Tested by: arrowd
Pull Request: https://github.com/freebsd/freebsd-ports/pull/273
Sponsored by: Serenity Cybersecurity, LLC