Made the latest XInput + Haptic changes compile and run on WinRT

Notes:
- Support for the 'Guide' button does not seem to be possible, as
  XInputGetStateEx is not available on WinRT.
- Haptic support appears to be working on WinRT now!
- SDL/WinRT does not allow calls to LoadLibrary or LoadLibraryEx.  The calls
  to those were removed by this change, but only when compiling for WinRT.
  Non-WinRT Windows will continue to detect and load XInput via LoadLibrary and
  GetProcAddress calls.