compositor: Initialize xserver variable
compositor: Initialize xserver variable

This variable is used unitialized if --xserver is not passed to weston.

Signed-off-by: Pekka Paalanen <6c0263d81461c956d504307112a4ce25eaacc3d1@gmail.com>

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