Fix debug script loader crashing missions

* open_script is now exposed in Script.h, perhaps it should be namespaced
as a static method on CTheScripts? I'm unsure what is preferred.
* I've moved the joypad code out of open_script to prevent buttons held
  down at mission load time from changing the scriptToLoad.

src/control/Script.h

@@ -591,5 +591,6 @@ void RetryMission(int, int);
 #endif
 
 #ifdef USE_DEBUG_SCRIPT_LOADER
+int open_script();
 extern int scriptToLoad;
 #endif