perf bpf_skel: Do not use typedef to avoid error on old clang

perf bpf_skel: Do not use typedef to avoid error on old clang

When building bpf_skel with clang-10, typedef causes confusions like:

  libbpf: map 'prev_readings': unexpected def kind var.

Fix this by removing the typedef.

Fixes: 74550c0e4667cb92 ("perf stat: Introduce 'bperf' to share hardware PMCs with BPF")
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Song Liu <songliubraving@fb.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lore.kernel.org/lkml/BEF5C312-4331-4A60-AEC0-AD7617CB2BC4@fb.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>