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

The manpage of poll(2) states that the prototype of poll is defined
in <poll.h>. Use that header file instead of <sys/poll.h> to allow
compilation against musl-libc.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>