Rewrite Rules: Check if $wp_rewrite->flush_rules() is callable before calling it in flush_rewrite_rules().

Props bsetiawan88, markjaquith.
Fixes #47087.
Built from https://develop.svn.wordpress.org/trunk@45699


git-svn-id: http://core.svn.wordpress.org/trunk@45510 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rewrite.php

@@ -273,7 +273,10 @@ function add_feed( $feedname, $function ) {
  */
 function flush_rewrite_rules( $hard = true ) {
 	global $wp_rewrite;
-	$wp_rewrite->flush_rules( $hard );
+
+	if ( is_callable( array( $wp_rewrite, 'flush_rules' ) ) ) {
+		$wp_rewrite->flush_rules( $hard );
+	}
 }
 
 /**