glsl: Construct gl_in with a location of -1.

glsl: Construct gl_in with a location of -1.

We use a location of -1 for variables which don't have their own
assigned locations--this includes ir_variables which represent named
interface blocks.  Technically the location assigned to gl_in doesn't
matter, since gl_in is only accessed via its members (which have their
own locations).  But it's nice to be consistent.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>