Added missing guards in implementation for PSP.

Thanks to Martin Gerhardy for pointing this out.

src/joystick/psp/SDL_sysjoystick.c

@@ -20,6 +20,8 @@
 */
 #include "../../SDL_internal.h"
 
+#if SDL_JOYSTICK_PSP
+
 /* This is the PSP implementation of the SDL joystick API */
 #include <pspctrl.h>
 #include <pspkernel.h>
@@ -263,5 +265,7 @@ SDL_JoystickGUID SDL_SYS_JoystickGetGUID(SDL_Joystick * joystick)
     return guid;
 }
 
+#endif /* SDL_JOYSTICK_PSP */
+
 /* vim: ts=4 sw=4
  */