libva: Add warning against major/minor libva updates

Add a single line comment (to ensure it's included in GitLab's context during automatic updates) to warn future maintainers of the risks of updating libva on a stable branch.

Apps built on top of the KDE runtime were failing to initialise libva after the last update (bb5fe838).

Due to the way libva loads drivers, updates are not forwards-compatible; the code responsible for loading a given driver will iterate backwards through all compatible libva versions to search for the init function matching __vaDriverInit_x_y.

When we published our runtime with the updated libva, along with published drivers in the GL extension built against that new libva, KDE apps were using the old libva to try to load these drivers, which would of course fail.

See #1579 (closed).