nested-client: include platform.h
nested-client: include platform.h

Missed because buliding nested clients requires cairo-glesv2.

Reviewed-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>

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