input/amijoy: Add missing platform check

On multi-platform kernels, the Amiga joystick driver may be initialized
when running on Amiga only. Else it may crash later.
Fortunately this driver is almost always compiled as a module (to avoid
conflicts with the mouse driver), so it needs an explicit insmod to
trigger a crash.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Cc: linux-input@vger.kernel.org