Throw an incorrect usage notice when the query argument of wpdb::prepare()...

Throw an incorrect usage notice when the query argument of wpdb::prepare() does not include a placeholder.

props ounziw.
fixes #25604.

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git-svn-id: https://core.svn.wordpress.org/trunk@26946 1a063a9b-81f0-0310-95a4-ce76da25c4cd