angstrom: add preferred provider for qt4

angstrom: add preferred provider for qt4

* bitbake had the following to say:

  NOTE: multiple providers are available for qt4x11 (qt4-x11-free-gles, qt4-x11-free);
  NOTE: consider defining PREFERRED_PROVIDER_qt4x11

  Which seems to give a fairly obvious way to fix it, so let's actually do so.

* I wish OE had something like PROVIDER_PREFERENCE = "1" to do this sort of stuff globally, but OTOH it really is a distro decision