vlc: fix shared library building

Fixes e.g.
http://autobuild.buildroot.net/results/9c7/9c7a97882d446a42cabcb07e4af7e6825192e70b



VLC cannot build static and shared libraries at the same time.

Also, live555 only has a static library, and VLC tries to link this into
a shared library. Therefore, disable the live555 plugin unless static
linking is selected. It would be better to build live555 as a shared
library, but that's for another patch.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>