libva: needs shared library support

Fixes:

  http://autobuild.buildroot.org/results/565/5655d1ee3f3b7503187d2e131d239cdde86683fb/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Bernd Kuhls <bernd.kuhls@t-online.de>