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/FileSystemUtils.cpp

@@ -127,7 +127,7 @@ int FILESYSTEM_init(char *argvZero, char* baseDir, char *assetsPath)
 	return 1;
 }
 
-void FILESYSTEM_deinit()
+void FILESYSTEM_deinit(void)
 {
 	if (PHYSFS_isInit())
 	{
@@ -135,12 +135,12 @@ void FILESYSTEM_deinit()
 	}
 }
 
-char *FILESYSTEM_getUserSaveDirectory()
+char *FILESYSTEM_getUserSaveDirectory(void)
 {
 	return saveDir;
 }
 
-char *FILESYSTEM_getUserLevelDirectory()
+char *FILESYSTEM_getUserLevelDirectory(void)
 {
 	return levelDir;
 }
@@ -209,7 +209,7 @@ void FILESYSTEM_mountassets(const char* path)
 	}
 }
 
-void FILESYSTEM_unmountassets()
+void FILESYSTEM_unmountassets(void)
 {
 	if (graphics.assetdir != "")
 	{
@@ -547,7 +547,7 @@ static void PLATFORM_copyFile(const char *oldLocation, const char *newLocation)
 	}
 }
 
-bool FILESYSTEM_openDirectoryEnabled()
+bool FILESYSTEM_openDirectoryEnabled(void)
 {
 	/* This is just a check to see if we're on a desktop or tenfoot setup.
 	 * If you're working on a tenfoot-only build, add a def that always