Add _() back to compat.php (it is non-default). see #16918. props aaroncampbell

git-svn-id: https://develop.svn.wordpress.org/trunk@17620 602fd350-edb4-49c9-b593-d223f7449a82

wp-includes/compat.php

@@ -0,0 +1,14 @@
+<?php
+/**
+ * WordPress implementation for PHP functions either missing from older PHP versions or not included by default.
+ *
+ * @package PHP
+ * @access private
+ */
+
+// If gettext isn't available
+if ( !function_exists('_') ) {
+	function _($string) {
+		return $string;
+	}
+}