Converted Visual Studio include paths to relative paths.

 - Include paths were specified with absolute paths, which break as soon as the project is moved to a different location. (e.g., when cloning to a new computer from GitHub).
 - Include paths are now specified relative to the project file, so everything should work as long as the entire XPC directory is present.
Merge pull request #9 from jwatkins42/master

Converted Visual Studio include paths to relative paths.