gnome-session: add dependency on libxtst6

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

This ends up in the package:

Package: gnome-session
Version: 2.32.1-r1
Depends: libx11-6 (>= 1.4.4), hicolor-icon-theme, libxrender1 (>= 0.9.7), libdbus-1-3 (>= 1.4.16), libdbus-glib-1-2 (>= 0.98), gconf, libxtst6 (>= 1.2.1),

But libxtst hasn't been built.

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