clients: fix an error message
clients: fix an error message

_GNU_SOURCE is always true as of c228e23b055f54d29f2df, so
program_invocation_short_name is available.

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