Bootstrap/Load: Correct sending the X-Pingback header in WP::send_headers().

The logic for sending the `X-Pingback` header for single posts that allow pings was recently moved from `WP::handle_404()`​ to a more appropriate place in `WP::send_headers()`.

To check whether pings are open for a particular post, that logic relies on the `$wp_query` global, which is declared in `WP::handle_404()`, but not in `WP::send_headers()`

This commit ensures that `$wp_query` is globalized in `WP::send_headers()` too, so that the check works as expected.

Follow-up to [54250].

Props strategio, sabernhardt, dlh, davidbaumwald, SergeyBiryukov.
Fixes #56840.

git-svn-id: https://develop.svn.wordpress.org/trunk@54636 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp.php

@@ -410,8 +410,12 @@ class WP {
 	 * @since 2.0.0
 	 * @since 4.4.0 `X-Pingback` header is added conditionally for single posts that allow pings.
 	 * @since 6.1.0 Runs after posts have been queried.
+	 *
+	 * @global WP_Query $wp_query WordPress Query object.
 	 */
 	public function send_headers() {
+		global $wp_query;
+
 		$headers       = array();
 		$status        = null;
 		$exit_required = false;