glsl: fix xfb_offset unsized array validation

glsl: fix xfb_offset unsized array validation

This partially fixes CTS test:
GL44-CTS.enhanced_layouts.xfb_get_program_resource_api

The test now fails at a tes evaluation shader with unsized output arrays.

The ARB_enhanced_layouts spec says:

   "It is a compile-time error to apply xfb_offset to the declaration of an
   unsized array."

So this seems like a bug in the CTS.

Reviewed-by: Dave Airlie <airlied@redhat.com>