configure.in: Disable joystick when disabling SDL

Currently, when using --disable-sdl, the Joystick support is still enabled by default. This leads to 
```
CPP     xbmc/input/ButtonTranslator.o
In file included from ButtonTranslator.cpp:44:0:
SDLJoystick.h:51:31: fatal error: SDL2/SDL_joystick.h: No such file or directory
```
Thus it does not make any sense to have joystick support enabled when the user wants to build without SDL.