tools: Synchronize tools/include/uapi/linux/bpf.h

Commit 747ea55e

 ("bpf: fix bpf_skb_in_cgroup helper naming") renames
BPF_FUNC_skb_in_cgroup to bpf_skb_under_cgroup, triggering this warning
while building perf:

  Warning: tools/include/uapi/linux/bpf.h differs from kernel

Update the copy to ack that, no changes needed, as
BPF_FUNC_skb_in_cgroup isn't used so far.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David Ahern <dsahern@gmail.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-x67d2gq8ct6ko12ex14q8bbx@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>