Recent Posts Widget: Use ob_end_flush() instead of ob_flush().

Merges [28195] to the 3.9 branch.

props m_i_n.
fixes #28009.

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git-svn-id: http://core.svn.wordpress.org/branches/3.9@28026 1a063a9b-81f0-0310-95a4-ce76da25c4cd