spec/attribute.dd: Specify possible trailing comma in LinkageAttribute

Author: CyberShadow

spec/attribute.dd

@@ -80,6 +80,7 @@ $(H2 $(LNAME2 linkage, Linkage Attribute))
 $(GRAMMAR
 $(GNAME LinkageAttribute):
     $(D extern) $(D $(LPAREN)) $(GLINK LinkageType) $(D $(RPAREN))
+    $(D extern) $(D $(LPAREN)) $(D C++) $(D ,) $(D $(RPAREN))
     $(D extern) $(D $(LPAREN)) $(D C++) $(D ,) $(GLINK2 type, QualifiedIdentifier) $(D $(RPAREN))
     $(D extern) $(D $(LPAREN)) $(D C++) $(D ,) $(GLINK NamespaceList) $(D $(RPAREN))