glsl: add missing explicit_stream flag to has_layout()

This will allow the ARB_shading_language_420pack rules in
glsl_parser.yy for catching duplicate layout qualifiers to be
triggered for the stream identifier rather than relying on the
code meant to catch duplicates within a single layout(...)

Reviewed-by: Samuel Iglesias Gonsálvez <siglesias@igalia.com>

src/glsl/ast_type.cpp

@@ -76,7 +76,8 @@ ast_type_qualifier::has_layout() const
           || this->flags.q.explicit_location
           || this->flags.q.explicit_index
           || this->flags.q.explicit_binding
-          || this->flags.q.explicit_offset;
+          || this->flags.q.explicit_offset
+          || this->flags.q.explicit_stream;
 }
 
 bool