Add a missing "REQUIRES: system-windows" to a Windows-only test.

This un-breaks builds on other platforms. Otherwise, they fail due to warnings like:

warning: unable to find a Visual Studio installation; try running Clang from a developer command prompt [-Wmsvc-not-found]

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@317716 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed