See the question and my original answer on StackOverflow

I had that problem myself, and I couldn't find the file anywhere on my disk.

Then I just realized the file seems to be included only in the Windows 8.1 SDK (and future SDKs to come I suppose), in the C:\Program Files (x86)\Windows Kits\8.1\Include\um path. The doc doesn't mention that (so you don't need Visual Studio 2013 to be able to use it)