libbpf: Add __percpu_kptr macro definition

Add __percpu_kptr macro definition in bpf_helpers.h.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20230827152800.1998492-1-yonghong.song@linux.dev


Signed-off-by: Alexei Starovoitov <ast@kernel.org>