libbpf: add xsk.h to install_headers target

The xsk.h header file was missing from the install_headers target in
the Makefile. This patch simply adds xsk.h to the set of installed
headers.

Fixes: 1cad0788 ("libbpf: add support for using AF_XDP sockets")
Reported-by: Bruce Richardson <bruce.richardson@intel.com>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>