External Libraries: Replace split() with explode() in the deprecated Atom and MagpieRSS files.

Before PHP7 `split()` was throwing a deprecated notice, in PHP7 the function is completely removed and throws a fatal error.

Props achbed.
Fixes #20673.

git-svn-id: https://develop.svn.wordpress.org/trunk@37402 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/atomlib.php

@@ -313,7 +313,7 @@ class AtomParser {
 
     function ns_to_prefix($qname, $attr=false) {
         # split 'http://www.w3.org/1999/xhtml:div' into ('http','//www.w3.org/1999/xhtml','div')
-        $components = split(":", $qname);
+        $components = explode(":", $qname);
 
         # grab the last one (e.g 'div')
         $name = array_pop($components);