perf tools: Do not put a variable sized type not at the end of a struct

perf tools: Do not put a variable sized type not at the end of a struct

As this is a GNU extension and while harmless in this case, we can do
the same thing in a more clearer way by using an existing thread_map
constructor.

With this we avoid this while compiling with clang:

  util/parse-events.c:2024:21: error: field 'map' with variable sized type 'struct thread_map' not at the end of a struct or class is a GNU extension
        [-Werror,-Wgnu-variable-sized-type-not-at-end]
                  struct thread_map map;
                                  ^
  1 error generated.

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-tqocbplnyyhpst6drgm2u4m3@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>