Explicitly declare void for all void parameter functions (#628)

Apparently in C, if you have `void test();`, it's completely okay to do
`test(2);`. The function will take in the argument, but just discard it
and throw it away. It's like a trash can, and a rude one at that. If you
declare it like `void test(void);`, this is prevented.

This is not a problem in C++ - doing `void test();` and `test(2);` is
guaranteed to result in a compile error (this also means that right now,
at least in all `.cpp` files, nobody is ever calling a void parameter
function with arguments and having their arguments be thrown away).
However, we may not be using C++ in the future, so I just want to lay
down the precedent that if a function takes in no arguments, you must
explicitly declare it as such.

I would've added `-Wstrict-prototypes`, but it produces an annoying
warning message saying it doesn't work in C++ mode if you're compiling
in C++ mode. So it can be added later.

desktop_version/src/GraphicsUtil.h

@@ -38,7 +38,7 @@ void FillRect( SDL_Surface* surface, SDL_Rect rect, int rgba );
 void ScrollSurface(SDL_Surface* _src, int pX, int py);
 
 SDL_Surface * FlipSurfaceVerticle(SDL_Surface* _src);
-void UpdateFilter();
+void UpdateFilter(void);
 SDL_Surface* ApplyFilter( SDL_Surface* _src );
 
 #endif /* GRAPHICSUTIL_H */