st/mesa: Enable the ARB_shading_language_420pack extension for 1.30+.

Any driver that supports GLSL 1.30 should be able to handle this
extension, as it's entirely implemented in the GLSL compiler.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Marek Olšák <maraeo@gmail.com>