x11/libXtst: update to 1.2.4

Remove GNU_CONFIGURE_MANPREFIX

Changes: https://gitlab.freedesktop.org/xorg/lib/libxtst/-/compare/libXtst-1.2.3...libXtst-1.2.4
Reviewed by: diizzy, jbeich
Tested by: arrowd
Pull Request: https://github.com/freebsd/freebsd-ports/pull/273
Sponsored by: Serenity Cybersecurity, LLC