LibC: Add <sys/poll.h> as an alias for <poll.h>

<poll.h> was originally <sys/poll.h> and some old software like X11
still uses the old path.

Userland/Libraries/LibC/sys/poll.h

@@ -0,0 +1,9 @@
+/*
+ * Copyright (c) 2021, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <poll.h>