AF_XDP: add xsk.{c,h} to Makefile and fix build

This patch makes sure we build AF_XDP-related code as part of libbpf. This
also required copying few uapi/linux headers and adding few used definitions
in include headers.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>

include/asm/barrier.h

@@ -0,0 +1,7 @@
+/* SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause) */
+#ifndef __ASM_BARRIER_H
+#define __ASM_BARRIER_H
+
+#include <linux/compiler.h>
+
+#endif