emulators/qemu31: unbreak with libglvnd >= 1.3.4

In file included from ui/egl-helpers.c:21:
include/ui/egl-helpers.h:46:55: error: unknown type name 'Window'
EGLSurface qemu_egl_init_surface_x11(EGLContext ectx, Window win);
                                                      ^
ui/egl-helpers.c:276:55: error: unknown type name 'Window'
EGLSurface qemu_egl_init_surface_x11(EGLContext ectx, Window win)
                                                      ^

PR:		258269
Reported by:	antoine (via exp-run)