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/GOGNetwork.c

@@ -6,16 +6,16 @@
 
 /* Totally unimplemented right now! */
 
-int32_t GOG_init()
+int32_t GOG_init(void)
 {
 	return 0;
 }
 
-void GOG_shutdown()
+void GOG_shutdown(void)
 {
 }
 
-void GOG_update()
+void GOG_update(void)
 {
 }