Fix E_STRICT notices in walkers. props dvarga. see #24356.

git-svn-id: http://core.svn.wordpress.org/trunk@24377 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix E_STRICT notice in wp_get_popular_importers(). see #24356.

git-svn-id: http://core.svn.wordpress.org/trunk@24378 1a063a9b-81f0-0310-95a4-ce76da25c4cd