compositor: add missing help text
compositor: add missing help text

The headless-backend.so was missing in available backend list

Signed-off-by: JoonCheol Park <05bb90bf014a89e8135e216dd97eba08627e02e6@gmail.com>
Reviewed-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

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