Include poll.h instead.

Fixes the following warning in systems with musl's libc:

  /usr/include/sys/poll.h:1:2: warning: #warning redirecting incorrect #include
  <sys/poll.h> to <poll.h> [-Wcpp]
Reported-by: John Spencer <maillist-linux@barfooze.de>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://thread.gmane.org/gmane.linux.kernel.perf.user/1687/focus=1690
Link: http://lkml.kernel.org/n/tip-k4ocrq1de3fk146oevy346bi@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>