weston_check_egl_extension() returns a bool, not a pointer.

Fixes: ce5b614c "clients/nested: use weston_check_egl_extension
                             over strstr"
Cc: Emil Velikov <emil.velikov@collabora.com>
Signed-off-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>