package/weston: do not select rpi-userland

rpi-userland is a provider for some virtual packages, so we can not
select it, as instructed in the manual:

    http://nightly.buildroot.org/#_infrastructure_for_virtual_packages



    ---8<---
    If your package really requires a specific provider, then you’ll
    have to make your package depends on this provider; you can not select a
    provider.
    ---8<---

Instead, just depend on it. Remove the comment as well.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>