Fixed the header guard in the darwin SDL_sysjoystick_c.h file.

src/joystick/darwin/SDL_sysjoystick_c.h

@@ -21,6 +21,7 @@
 #include "../../SDL_internal.h"
 
 #ifndef SDL_JOYSTICK_IOKIT_H
+#define SDL_JOYSTICK_IOKIT_H
 
 #include <IOKit/hid/IOHIDLib.h>