Libraries: Create top level directory for libraries.

Things were getting a little crowded in the project root, so this patch
moves the Lib*/ directories into Libraries/.

Libraries/LibC/poll.h

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+#define POLLIN (1u << 0)
+#define POLLPRI (1u << 2)
+#define POLLOUT (1u << 3)
+#define POLLERR (1u << 4)
+#define POLLHUP (1u << 5)
+#define POLLNVAL (1u << 6)
+
+struct pollfd {
+    int fd;
+    short events;
+    short revents;
+};
+
+int poll(struct pollfd* fds, int nfds, int timeout);
+
+__END_DECLS