perf bpf: Check libbpf version to use btf_dump_type_data_opts.emit_strings

perf bpf: Check libbpf version to use btf_dump_type_data_opts.emit_strings

When building perf with LIBBPF_DYNAMIC=1 on a fedora system with
libbpf-devel 1.5 I it was breaking with:

util/bpf-event.c: In function ‘format_btf_variable’:
util/bpf-event.c:291:18: error: ‘const struct btf_dump_type_data_opts’ has no member named ‘emit_strings’
  291 |                 .emit_strings = 1,
      |                  ^~~~~~~~~~~~
util/bpf-event.c:291:33: error: initialized field overwritten [-Werror=override-init]
  291 |                 .emit_strings = 1,
      |                                 ^
util/bpf-event.c:291:33: note: (near initialization for ‘opts.skip_names’)

Check the version before using that feature.

Reviewed-by: Ian Rogers <irogers@google.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: James Clark <james.clark@linaro.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>