joy.cpl: Move dinput test tab logic to dinput.c.

dlls/joy.cpl/joy_private.h

@@ -30,7 +30,7 @@
 
 #include "resource.h"
 
-extern void set_di_device_state( HWND hwnd, DIJOYSTATE *state, DIDEVCAPS *caps );
+extern INT_PTR CALLBACK test_di_dialog_proc( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam );
 extern LRESULT CALLBACK test_di_axes_window_proc( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam );
 extern LRESULT CALLBACK test_di_povs_window_proc( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam );
 extern LRESULT CALLBACK test_di_buttons_window_proc( HWND hwnd, UINT msg, WPARAM wparam, LPARAM lparam );