Don't include pingback unnecessarily

See https://core.trac.wordpress.org/ticket/20226 and these commits:
https://core.trac.wordpress.org/changeset/34442
https://core.trac.wordpress.org/changeset/34443

Author: staylor

header.php

@@ -14,7 +14,9 @@
 	<meta charset="<?php bloginfo( 'charset' ); ?>">
 	<meta name="viewport" content="width=device-width, initial-scale=1">
 	<link rel="profile" href="http://gmpg.org/xfn/11">
+	<?php if ( is_singular() && pings_open( get_queried_object() ) ): ?>
 	<link rel="pingback" href="<?php bloginfo( 'pingback_url' ); ?>">
+	<?php endif; ?>
 	<?php wp_head(); ?>
 </head>