perf auxtrace: Add missing 'struct perf_sample' forward declaration

Its needed, was being obtained indirectly, fix it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-c3k1il7sm28old4e22nwlm7l@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>