GdkDevice: Fix an inverted assertion
GdkDevice: Fix an inverted assertion

gdk_device_list_slave_devices only makes sense to call on master
devices, yet its g_return_if_fail check made it reject such devices.

Pointed out by monty.

Lyude/gtk-,Sidnioulz/SandboxGtk,Distrotech/gtk2,alexlarsson/gtk,Lyude/gtk-,chergert/gtk,davidgumberg/gtk,davidgumberg/gtk,davidgumberg/gtk,alexlarsson/gtk,Distrotech/gtk2,Distrotech/gtk2,chergert/gtk,grubersjoe/adwaita,chergert/gtk,ebassi/gtk,msteinert/gtk,davidgumberg/gtk,Sidnioulz/SandboxGtk,Sidnioulz/SandboxGtk,jigpu/gtk,jadahl/gtk,alexlarsson/gtk,Adamovskiy/gtk,msteinert/gtk,jadahl/gtk,ahodesuka/gtk,davidt/gtk,jigpu/gtk,ebassi/gtk,grubersjoe/adwaita,jessevdk/gtk,Lyude/gtk-,jigpu/gtk,Lyude/gtk-,davidt/gtk,alexlarsson/gtk,Adamovskiy/gtk,grubersjoe/adwaita,jessevdk/gtk,jigpu/gtk,Adamovskiy/gtk,jigpu/gtk,chergert/gtk,Adamovskiy/gtk,jessevdk/gtk,Sidnioulz/SandboxGtk,Sidnioulz/SandboxGtk,jigpu/gtk,ahodesuka/gtk,msteinert/gtk,ebassi/gtk,jessevdk/gtk,grubersjoe/adwaita,bratsche/gtk-,grubersjoe/adwaita,Adamovskiy/gtk,davidt/gtk,jadahl/gtk,Distrotech/gtk2,Distrotech/gtk2,ebassi/gtk,chergert/gtk,jadahl/gtk,ebassi/gtk,alexlarsson/gtk,ahodesuka/gtk,Adamovskiy/gtk,bratsche/gtk-,bratsche/gtk-,ahodesuka/gtk,davidt/gtk,ahodesuka/gtk,Adamovskiy/gtk,msteinert/gtk,davidgumberg/gtk,jessevdk/gtk,davidgumberg/gtk,Lyude/gtk-,Lyude/gtk-,msteinert/gtk,chergert/gtk,Adamovskiy/gtk,jessevdk/gtk,alexlarsson/gtk,Lyude/gtk-,grubersjoe/adwaita,bratsche/gtk-,ebassi/gtk,ahodesuka/gtk,davidgumberg/gtk,alexlarsson/gtk,jadahl/gtk,jadahl/gtk,ahodesuka/gtk,alexlarsson/gtk,jadahl/gtk,Lyude/gtk-,ahodesuka/gtk,jessevdk/gtk,msteinert/gtk,Sidnioulz/SandboxGtk,davidt/gtk,jigpu/gtk,chergert/gtk,bratsche/gtk-,Distrotech/gtk2,davidgumberg/gtk,jigpu/gtk,davidt/gtk,grubersjoe/adwaita,chergert/gtk,jadahl/gtk,bratsche/gtk-,grubersjoe/adwaita