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

`ob_end_flush()` flushes the output buffer *and* turns output buffering off, same as `ob_get_flush()`.

props m_i_n.
see #28009 for trunk.


git-svn-id: https://develop.svn.wordpress.org/trunk@28217 602fd350-edb4-49c9-b593-d223f7449a82