Include <poll.h> instead of <sys/poll.h>

According to POSIX.1-2001, the correct include path is <poll.h>. And
when compiling against musl, it generates "deprecated" warnings, if
<sys/poll.h> is included.

Signed-off-by: Felix Hädicke <felixhaedicke@web.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>