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/Screen.h

@@ -9,25 +9,25 @@ class Screen
 {
 public:
 	void init(const ScreenSettings& settings);
-	void destroy();
+	void destroy(void);
 
 	void GetSettings(ScreenSettings* settings);
 
-	void LoadIcon();
+	void LoadIcon(void);
 
 	void ResizeScreen(int x, int y);
-	void ResizeToNearestMultiple();
+	void ResizeToNearestMultiple(void);
 	void GetWindowSize(int* x, int* y);
 
 	void UpdateScreen(SDL_Surface* buffer, SDL_Rect* rect);
-	void FlipScreen();
+	void FlipScreen(void);
 
-	const SDL_PixelFormat* GetFormat();
+	const SDL_PixelFormat* GetFormat(void);
 
-	void toggleFullScreen();
-	void toggleStretchMode();
-	void toggleLinearFilter();
-	void resetRendererWorkaround();
+	void toggleFullScreen(void);
+	void toggleStretchMode(void);
+	void toggleLinearFilter(void);
+	void resetRendererWorkaround(void);
 
 	bool isWindowed;
 	bool isFiltered;