libbpf: Prevent deprecation warnings in xsk.c

xsk.c is using own APIs that are marked for deprecation internally.
Given xsk.c and xsk.h will be gone in libbpf 1.0, there is no reason to
do public vs internal function split just to avoid deprecation warnings.
So just add a pragma to silence deprecation warnings (until the code is
removed completely).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211124193233.3115996-4-andrii@kernel.org