Add sstream header to NetworkProxyWin

Fixes issue [#10494](http://trac.mantidproject.org/mantid/ticket/10494)

Tester: Check http://builds.mantidproject.org/view/Develop%20Builds/job/develop_clean_win7_debug/. It now builds but one of the tests is failing. However, the failing test is related to [#10494](http://trac.mantidproject.org/mantid/ticket/10482) and will be fixed there.