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 <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Wang Nan <[e-mail]>
Link: http://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/tests/fdarray.c

···
 #include <api/fd/array.h>
+#include <poll.h>
 #include "util/debug.h"
 #include "tests/tests.h"