gnome-settings-daemon: add dependency on libxtst6

It's autodetected and leads to sstate based breakage on autobuilders:

This ends up in the package:

Package: gnome-settings-daemon
Version: 2.32.1-r3
Depends: libltdl7 (>= 2.4.2), hicolor-icon-theme, libjson0 (>= 0.9), libxcb1 (>= 1.8.1), libxrender1 (>= 0.9.7), libpixman-1-0 (>= 0.25.2), libxml2 (>= 2.7.8), libxext6 (>= 1.3.1), libz1 (>= 1.2.6), libxtst6 (>= 1.2.1),

But libxtst hasn't been built.

Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>