Cron: Fix undefined variable notice.

Restore `$gmt_time` variable removed in error from [44483].

Props afragen.
Fixes #45870.



git-svn-id: https://develop.svn.wordpress.org/trunk@44505 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-cron.php

@@ -76,6 +76,8 @@ if ( empty( $crons ) ) {
 	die();
 }
 
+$gmt_time = microtime( true );
+
 // The cron lock: a unix timestamp from when the cron was spawned.
 $doing_cron_transient = get_transient( 'doing_cron' );