tablet-shell: Fix copy and paste error in unlock handler
tablet-shell: Fix copy and paste error in unlock handler

The signal handler was using the wrong member to find the containing
structure of the unlock listener.

https://bugs.freedesktop.org/show_bug.cgi?id=57637

mchalupa/weston,udoprog/weston,kwm81/weston,sir-murray/weston,xorgy/weston,udoprog/weston,giucam/weston,Fantu/compositor-spice,jonnylamb/weston,mchalupa/weston,Gnurou/weston,Fantu/compositor-spice,krezovic/weston,Tarnyko/weston-xdg_surface_present,sir-murray/weston,eyolfson/weston,eyolfson/weston,Fantu/compositor-spice,Tarnyko/weston-xdg_surface_present,krezovic/weston,xorgy/weston,giucam/weston,Gnurou/weston,kwm81/weston,jonnylamb/weston