Yet another one undefined comparison of 'const' and a 'non-const' iterators

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

Change-Id: I7b4ba07ebe51c73893a3d6b77dcf5681b7638efb
Signed-off-by: Stephan Bergmann <sbergman@redhat.com>