bpftool: Fix generated code in codegen_asserts

Arnaldo reported perf compilation fail with:

  $ make -k BUILD_BPF_SKEL=1 CORESIGHT=1 PYTHON=python3
  ...
  In file included from util/bpf_counter.c:28:
  /tmp/build/perf//util/bpf_skel/bperf_leader.skel.h: In function ‘bperf_leader_bpf__assert’:
  /tmp/build/perf//util/bpf_skel/bperf_leader.skel.h:351:51: error: unused parameter ‘s’ [-Werror=unused-parameter]
    351 | bperf_leader_bpf__assert(struct bperf_leader_bpf *s)
        |                          ~~~~~~~~~~~~~~~~~~~~~~~~~^
  cc1: all warnings being treated as errors

If there's nothing to generate in the new assert function,
we will get unused 's' warn/error, adding 'unused' attribute to it.

Fixes: 08d4dba6

 ("bpftool: Bpf skeletons assert type sizes")
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Link: https://lore.kernel.org/bpf/20220328083703.2880079-1-jolsa@kernel.org