Adjusted KnownTypeNames to handle TypeSystemExtensionDefinition

src/main/scala/sangria/validation/rules/KnownTypeNames.scala

@@ -15,7 +15,7 @@ import sangria.validation._
 class KnownTypeNames extends ValidationRule {
   override def visitor(ctx: ValidationContext) = new AstValidatingVisitor {
     override val onEnter: ValidationVisit = {
-      case _: ast.ObjectTypeDefinition | _: ast.InterfaceTypeDefinition | _: ast.UnionTypeDefinition | _: ast.InputObjectTypeDefinition | _: ast.TypeExtensionDefinition | _: ast.SchemaDefinition ⇒
+      case _: ast.ObjectTypeDefinition | _: ast.InterfaceTypeDefinition | _: ast.UnionTypeDefinition | _: ast.InputObjectTypeDefinition | _: ast.TypeSystemExtensionDefinition | _: ast.SchemaDefinition ⇒
         // When validating SDL, at the moment schema does not know about these types.
         // All type names are validated in the schema materializer as new schema is constructed.
         AstVisitorCommand.RightSkip