Mark VVV_exit implementation as VVV_NORETURN

For consistency. And in case it matters that much (which I don't really
think it does).

desktop_version/src/main.cpp

@@ -708,7 +708,7 @@ static void cleanup(void)
     FILESYSTEM_deinit();
 }
 
-void VVV_exit(const int exit_code)
+VVV_NORETURN void VVV_exit(const int exit_code)
 {
     cleanup();
     exit(exit_code);