BuildSystem: Move all userpace libraries under the userspace directory

As the number of libraries is increasing, root directory starts to
expand. This adds better organization for libraries

userspace/libraries/LibC/include/poll.h

@@ -0,0 +1,34 @@
+#ifndef _POLL_H
+#define _POLL_H 1
+
+// https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/poll.h.html
+
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+
+struct pollfd
+{
+	int		fd;			/* The following descriptor being polled. */
+	short	events;		/* The input event flags. */
+	short	revents;	/* The output event flags. */
+};
+
+typedef unsigned long nfds_t;
+
+#define POLLIN		0x001
+#define POLLRDNORM	0x002
+#define POLLRDBAND	0x004
+#define POLLPRI		0x008
+#define POLLOUT		0x010
+#define POLLWRNORM	0x020
+#define POLLWRBAND	0x040
+#define POLLERR		0x080
+#define POLLHUP		0x100
+#define POLLNVAL	0x200
+
+int poll(struct pollfd fds[], nfds_t nfds, int timeout);
+
+__END_DECLS
+
+#endif