As this is where we expect to find bpf/bpf_helpers.h, etc.

This needs more work to make it follow LIBBPF_DYNAMIC=1 usage, i.e. when
not using the system libbpf it should use the headers in the in-kernel
sources libbpf in tools/lib/bpf.

We need to do that anyway to avoid this mixup system libbpf and
in-kernel files, so we'll get this sorted out that way.

And this also may become moot as we move to using BPF skels for this
feature.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>