trying to fix Windows breaker into the blind

- ring.hxx(246) : error C2440: ´<function-style-cast>´ : cannot convert
  from ´const SwViewShell *´ to ´sw::RingIterator<const value_type>´
- ring.hxx(250) : error C2665: ´sw::RingIterator<const
  value_type>::RingIterator´ : none of the 3 overloads could convert all
  the argument types
- only tested on gcc-4.8 here

Change-Id: I181b8a694834133ef31a4a89bf93c90e3ce63b54
Reviewed-on: https://gerrit.libreoffice.org/13334
Reviewed-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>
Tested-by: Björn Michaelsen <bjoern.michaelsen@canonical.com>