glsl: mark special built-in inputs referenced by vertex stage

glsl: mark special built-in inputs referenced by vertex stage

Refactoring done on active attribute queries did not take in to
account special built-in inputs for the vertex stage. This commit
sets them referenced by vertex stage so that they get enumerated
properly.

Fixes Piglit test 'get-active-attrib-returns-all-inputs' failure.

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90243
Acked-by: Jose Fonseca <jfonseca@vmware.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>
Reviewed-By: Martin Peres <martin.peres@linux.intel.com>