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.
Built from https://develop.svn.wordpress.org/trunk@54636


git-svn-id: http://core.svn.wordpress.org/trunk@54188 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.1-RC1-54635';
+$wp_version = '6.1-RC1-54636';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.