Fix warnings about static function and prototype

test/checkkeysthreads.c

@@ -25,7 +25,7 @@
 #include <SDL3/SDL.h>
 #include <SDL3/SDL_main.h>
 
-int done;
+static int done;
 
 /* Call this instead of exit(), so we can clean up SDL: atexit() is evil. */
 static void
@@ -103,7 +103,7 @@ print_modifiers(char **text, size_t *maxlen)
 }
 
 static void
-PrintModifierState()
+PrintModifierState(void)
 {
     char message[512];
     char *spot;
@@ -162,7 +162,7 @@ PrintText(const char *eventtype, const char *text)
     SDL_Log("%s Text (%s): \"%s%s\"\n", eventtype, expanded, *text == '"' ? "\\" : "", text);
 }
 
-void loop()
+static void loop(void)
 {
     SDL_Event event;
     /* Check for events */