Discovered that the SDK generated for Windows Embedded Compact 2013 to be used in Visual Studio 2013 was missing the ddk\inc content.
Some of the products we build with this SDK include headers from this space, particularly, devload.h and nuiouser.h.
Was able to resolve this issue, and get all of the ddk\inc headers in the SDK by editing the SDK properties.
I used the Additional Folders dialog to do this.
Click Add... button For Source folder
Use browse ("...") button to navigate to "C:\WINCE800\public\common\ddk\inc"
For "Target:" enter: SDK\inc
(You do not need to check the Include Subfolders checkbox as ddk\inc has no subfolders.)
Build SDK ddk\inc
headers should now be included in your SDK.