glsl: allow default qualifiers for shader storage block definitions

glsl: allow default qualifiers for shader storage block definitions

This kind of definitions:

    layout(xxx) buffer;

was not supported by commit 84fc5fece006.

Signed-off-by: Samuel Iglesias Gonsalvez <siglesias@igalia.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>