tablet-shell: Fix compiling error for using stderr.
tablet-shell: Fix compiling error for using stderr.

Not including the stdio.h makes this compiling error.

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