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>

src/compiler/glsl/ir.h

@@ -2621,6 +2621,9 @@ is_gl_identifier(const char *s)
    return s && s[0] == 'g' && s[1] == 'l' && s[2] == '_';
 }
 
+const glsl_type *
+get_varying_type(const ir_variable *var, gl_shader_stage stage);
+
 extern "C" {
 #endif /* __cplusplus */