perf tests: Synthesize struct instead of using field after variable sized type

perf tests: Synthesize struct instead of using field after variable sized type

End result is the same, its an ABI, so the struct won't change, avoid
using a GNU extension, so that we can catch other cases that may be bugs.

Caught when building with clang:

  tests/parse-no-sample-id-all.c:53:20: error: field 'attr' with variable sized type 'struct attr_event' not at the end of a struct or class is a GNU extension
        [-Werror,-Wgnu-variable-sized-type-not-at-end]
          struct attr_event attr;
                            ^
  1 error generated.

Testing it:

  # perf test sample_id
  24: Parse with no sample_id_all bit set        : Ok
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-e2vs1x771fc208uvxnwcf08b@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>