rawinput: add basic support for xinput (#617)

## Link to GitHub Issue or related Pull Request, if one exists
#616 

## Description of change
Adds preliminary support for XInput controllers. Enough to manually bind
buttons and map analogs.

src/spice2x/rawinput/device.h

@@ -31,7 +31,8 @@ namespace rawinput {
         SEXTET_OUTPUT,
         PIUIO_DEVICE,
         SMX_STAGE,
-        SMX_DEDICAB
+        SMX_DEDICAB,
+        XINPUT_GAMEPAD,
     };
 
     enum MouseKeys {