LibPthread: Fix typo in pthread_mutex_lock()

We were casting the pthread_mutex_t* instead of pthread_mutex_t::lock
to an Atomic<u32>. This still worked fine, since "lock" is the first
member of pthread_mutex_t.

Libraries/LibPthread/pthread.cpp

@@ -114,7 +114,7 @@ int pthread_mutex_destroy(pthread_mutex_t*)
 
 int pthread_mutex_lock(pthread_mutex_t* mutex)
 {
-    auto* atomic = reinterpret_cast<Atomic<u32>*>(mutex);
+    auto* atomic = reinterpret_cast<Atomic<u32>*>(mutex->lock);
     pthread_t this_thread = pthread_self();
     for (;;) {
         u32 expected = false;