fix: C standard requires prototype (#807)

Unable to compile on Mac without a static cast to ensure the return
type of the thread id.

tests/integration/t_local_cache_threading.c

@@ -82,7 +82,7 @@ struct rand_state {
 #define RNG_MAX (RNG_MODULUS - 1)
 
 static unsigned long threadid_get_callback() {
-    return aws_thread_current_thread_id();
+    return (unsigned long)aws_thread_current_thread_id();
 }
 
 static struct aws_mutex *mutex_array = NULL;