[all] use explicit void parameter lists

This makes it a compile-time error to call a function that semantically
takes no parameters with a nonzero number of arguments.

Previously, such code would still compile, but risk blowing up the stack
if a compiler chose to use something other than caller-cleanup calling
conventions.

host/platform/Linux/src/platform.c

@@ -54,18 +54,18 @@ void sigHandler(int signo)
   app_quit();
 }
 
-bool app_init()
+bool app_init(void)
 {
   signal(SIGINT, sigHandler);
   return true;
 }
 
-const char * os_getExecutable()
+const char * os_getExecutable(void)
 {
   return app.executable;
 }
 
-const char * os_getDataPath()
+const char * os_getDataPath(void)
 {
   return app.dataPath;
 }