libbpf: remove likely/unlikely in xsk.h

This patch removes the use of likely and unlikely in xsk.h since they
create a dependency on Linux headers as reported by several
users. There have also been reports that the use of these decreases
performance as the compiler puts the code on two different cache lines
instead of on a single one. All in all, I think we are better off
without them.

Fixes: 1cad0788

 ("libbpf: add support for using AF_XDP sockets")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>