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/preloader.cpp

@@ -11,7 +11,7 @@ static int pre_darkcol=0, pre_lightcol=0, pre_curcol=0, pre_coltimer=0, pre_offs
 static int pre_frontrectx=30, pre_frontrecty=20, pre_frontrectw=260, pre_frontrecth=200;
 static int pre_temprectx=0, pre_temprecty=0, pre_temprectw=320, pre_temprecth=240;
 
-void preloaderinput()
+void preloaderinput(void)
 {
   game.press_action = false;
 
@@ -26,7 +26,7 @@ void preloaderinput()
   }
 }
 
-void preloaderrenderfixed()
+void preloaderrenderfixed(void)
 {
   if (pre_transition < 30) pre_transition--;
   if(pre_transition>=30){
@@ -49,7 +49,7 @@ void preloaderrenderfixed()
   }
 }
 
-void preloaderrender()
+void preloaderrender(void)
 {
   if(pre_transition>=30){
     switch(pre_curcol) {