add missing `reserved` field to struct SDL_JoyBallEvent,

Fixes https://github.com/libsdl-org/SDL/issues/9327

include/SDL3/SDL_events.h

@@ -343,6 +343,7 @@ typedef struct SDL_MouseMotionEvent
 typedef struct SDL_JoyBallEvent
 {
     Uint32 type;        /**< ::SDL_JOYBALLMOTION */
+    Uint32 reserved;
     Uint64 timestamp;   /**< In nanoseconds, populated using SDL_GetTicksNS() */
     SDL_JoystickID which; /**< The joystick instance id */
     Uint8 ball;         /**< The joystick trackball index */