Customize: Restore search icon to available widgets pane incorrectly removed in [41893].

See #42215.
Fixes #42472 for trunk.

Built from https://develop.svn.wordpress.org/trunk@42132


git-svn-id: https://core.svn.wordpress.org/trunk@41963 1a063a9b-81f0-0310-95a4-ce76da25c4cd