winsys-egl: fix typo in _cogl_winsys_onscreen_init
winsys-egl: fix typo in _cogl_winsys_onscreen_init

We were trying to set attr.event_mask instead of xattr.event mask which
was causing a compilation error.

djdeath/cogl,gcampax/cogl,djdeath/cogl-android,Distrotech/cogl,Distrotech/cogl,collects/cogl,spatulasnout/cogl,djdeath/cogl-android,gcampax/cogl,djdeath/cogl,spatulasnout/cogl,collects/cogl,gcampax/cogl,djdeath/cogl,Distrotech/cogl,djdeath/cogl-android,gcampax/cogl,Distrotech/cogl