Can't use const_reverse_iterator here.

Fix build error:
    no match for ‘operator!=’ in ‘it != std::vector<_Tp, _Alloc>::rend()
    [with _Tp = rtl::OUString, _Alloc = std::allocator<rtl::OUString>]()’

Change-Id: I4d20a68259163303ce7f14014c939cc7dbdf2784
Reviewed-on: https://gerrit.libreoffice.org/7145
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>