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/Music.h

@@ -11,19 +11,19 @@
 class musicclass
 {
 public:
-	musicclass();
-	void init();
-	void destroy();
+	musicclass(void);
+	void init(void);
+	void destroy(void);
 
 	void play(int t, const double position_sec = 0.0, const int fadein_ms = 3000);
 	void resume(const int fadein_ms = 0);
-	void haltdasmusik();
-	void silencedasmusik();
+	void haltdasmusik(void);
+	void silencedasmusik(void);
 	void fadeMusicVolumeIn(int ms);
 	void fadeout(const bool quick_fade_ = true);
-	void fadein();
-	void processmusicfadein();
-	void processmusic();
+	void fadein(void);
+	void processmusicfadein(void);
+	void processmusic(void);
 	void niceplay(int t);
 
 	void changemusicarea(int x, int y);