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/RenderFixed.cpp

@@ -7,7 +7,7 @@
 #include "Script.h"
 #include "UtilityClass.h"
 
-void titleupdatetextcol()
+void titleupdatetextcol(void)
 {
     graphics.col_tr = graphics.titlebg.r - (help.glow / 4) - int(fRandom() * 4);
     graphics.col_tg = graphics.titlebg.g - (help.glow / 4) - int(fRandom() * 4);
@@ -20,7 +20,7 @@ void titleupdatetextcol()
     if(graphics.col_tb>255) graphics.col_tb=255;
 }
 
-void gamerenderfixed()
+void gamerenderfixed(void)
 {
     if (!game.blackout && !game.completestop)
     {
@@ -154,7 +154,7 @@ void gamerenderfixed()
 #endif
 }
 
-void titlerenderfixed()
+void titlerenderfixed(void)
 {
     if (!game.colourblindmode)
     {
@@ -182,7 +182,7 @@ void titlerenderfixed()
     }
 }
 
-void maprenderfixed()
+void maprenderfixed(void)
 {
     graphics.updatetextboxes();
     graphics.updatetitlecolours();
@@ -248,7 +248,7 @@ void maprenderfixed()
     }
 }
 
-void gamecompleterenderfixed()
+void gamecompleterenderfixed(void)
 {
     graphics.updatetitlecolours();