added missing changedisplayname.php

settings/ajax/changedisplayname.php

+<?php
+// Check if we are a user
+OCP\JSON::callCheck();
+OC_JSON::checkLoggedIn();
+
+$username = isset($_POST["username"]) ? $_POST["username"] : OC_User::getUser();
+$displayName = $_POST["displayName"];
+
+// Return Success story
+if( OC_User::setDisplayName( $username, $displayName )) {
+	OC_JSON::success(array("data" => array( "username" => $username )));
+}
+else{
+	OC_JSON::error(array("data" => array( "message" => "Unable to change display name" )));
+}
\ No newline at end of file