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/linux/kernel.h

@@ -39,5 +39,6 @@
 #endif
 
 #define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
+#define __KERNEL_DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
 
 #endif