LibPthread: Stub out pthread_atfork()

Author: https://github.com/alimpfard
Commit: 2e50c232f7
Pull-request: https://github.com/SerenityOS/serenity/pull/5325
Issue: https://github.com/SerenityOS/serenity/issues/90
Reviewed-by: https://github.com/BenWiederhake
Reviewed-by: https://github.com/linusg
Reviewed-by: https://github.com/sunverwerth

Userland/Libraries/LibPthread/pthread.cpp

@@ -740,4 +740,9 @@ int pthread_rwlockattr_setpshared(pthread_rwlockattr_t*, int)
     ASSERT_NOT_REACHED();
 }
 
+int pthread_atfork(void (*)(void), void (*)(void), void (*)(void))
+{
+    ASSERT_NOT_REACHED();
+}
+
 } // extern "C"