perf test fdarray: Add missing poll.h header

It uses poll() but was getting the needed header by chance, do it
explicitely.

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-76b3c5imnl6p69j4lqewzu9l@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>