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/Tower.h

@@ -4,7 +4,7 @@
 class towerclass
 {
 public:
-    towerclass();
+    towerclass(void);
 
     int backat(int xp, int yp, int yoff);
 
@@ -12,13 +12,13 @@ public:
 
     int miniat(int xp, int yp, int yoff);
 
-    void loadminitower1();
+    void loadminitower1(void);
 
-    void loadminitower2();
+    void loadminitower2(void);
 
-    void loadbackground();
+    void loadbackground(void);
 
-    void loadmap();
+    void loadmap(void);
 
     short back[40 * 120];
     short contents[40 * 700];