x11/libXi: update to 1.8.1

Remove GNU_CONFIGURE_MANPREFIX

Changes: https://gitlab.freedesktop.org/xorg/lib/libxi/-/compare/libXi-1.8...libXi-1.8.1
Reviewed by: diizzy, jbeich
Tested by: arrowd
Pull Request: https://github.com/freebsd/freebsd-ports/pull/273
Sponsored by: Serenity Cybersecurity, LLC