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/Ent.cpp

@@ -3,12 +3,12 @@
 #include "Game.h"
 #include "Graphics.h"
 
-entclass::entclass()
+entclass::entclass(void)
 {
 	clear();
 }
 
-void entclass::clear()
+void entclass::clear(void)
 {
 	invis = false;
 	type = 0;
@@ -65,7 +65,7 @@ void entclass::clear()
 	lerpoldyp = 0;
 }
 
-bool entclass::outside()
+bool entclass::outside(void)
 {
 	// Returns true if any point of the entity is outside the map.
 	// Adjusts velocity for a clean collision.
@@ -603,7 +603,7 @@ void entclass::settreadmillcolour( int rx, int ry )
 	}
 }
 
-void entclass::updatecolour()
+void entclass::updatecolour(void)
 {
 	switch (size)
 	{
@@ -650,7 +650,7 @@ void entclass::updatecolour()
 	}
 }
 
-bool entclass::ishumanoid()
+bool entclass::ishumanoid(void)
 {
 	return type == 0
 		|| type == 12