[msvc] Add a definition for _WIN32_WINNT to MonoPosixHelper

MonoPosixHelper fails to build in my testing, due to config.h bailing the
build out if it doesn't think the build platform is XP SP2 or higher.
Adding this define forces that minimum Windows version, allowing mono.sln
to build fully.
Merge pull request #1417 from directhex/fix_msvc_build

[msvc] Add a definition for _WIN32_WINNT to MonoPosixHelper