perf tools: Remove FLEX_ARRAY definition

perf tools: Remove FLEX_ARRAY definition

We rely on symbol->name[0] since the beginning of tools/perf/, never
having received any complaint about it, also all the containers build
perf just fine, so remove this git codebase remnant.

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-jsjpgojut8e22o2gtz83augk@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>