should be comparing the values of the elements, not the pointers.

Regression from
    commit 92744735
    Date:   Sat Sep 12 16:49:29 2015 -0600
    replace boost::ptr_container with std::container<std::unique_ptr>

noticed while doing some other plugin work

Change-Id: I96230e52571e9a96e1c96f53f61bbc5afe902b31
Reviewed-on: https://gerrit.libreoffice.org/68738
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>