Fix bug in TSAB: readonly and ref modifiers should only be added to structs if ShowModifiers == true.

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -1016,12 +1016,14 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				case TypeKind.Struct:
 					classType = ClassType.Struct;
 					modifiers &= ~Modifiers.Sealed;
+					if (ShowModifiers) {
 						if (typeDefinition.IsReadOnly) {
 							modifiers |= Modifiers.Readonly;
 						}
 						if (typeDefinition.IsByRefLike) {
 							modifiers |= Modifiers.Ref;
 						}
+					}
 					break;
 				case TypeKind.Enum:
 					classType = ClassType.Enum;