Fix menu.cpp build when max_joysticks=0

menu.cpp always needs SDL.h, but included it only as a side effect of
joy.h including SDL.h.  joy.h only includes SDL.h when max_joysticks!=0.

Reported-by: kreatordxx <https://github.com/dxx-rebirth/dxx-rebirth/issues/258>

similar/main/menu.cpp

@@ -25,6 +25,7 @@ COPYRIGHT 1993-1999 PARALLAX SOFTWARE CORPORATION.  ALL RIGHTS RESERVED.
 
 #include <stdio.h>
 #include <string.h>
+#include <SDL.h>
 
 #include "menu.h"
 #include "inferno.h"