libbpf: remove compile time warning from libbpf_util.h

Having a helpful compile time warning in libbpf_util.h is not a good
idea since all warnings are treated as errors. Change this into a
comment in the code instead.

Fixes: b7e3a280

 ("libbpf: remove dependency on barrier.h in xsk.h")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>