fix undefined behavior in ma_thread_wait__posix

Quoting man for pthread_detach:
> The behavior is undefined if the value specified by the thread argument
> to pthread_detach() does not refer to a joinable thread.

Since the thread was joined right before, pthread_detach should not be called.

miniaudio.h

@@ -15534,7 +15534,6 @@ static ma_result ma_thread_create__posix(ma_thread* pThread, ma_thread_priority
 static void ma_thread_wait__posix(ma_thread* pThread)
 {
     pthread_join((pthread_t)*pThread, NULL);
-    pthread_detach((pthread_t)*pThread);
 }