Include poll.h instead of sys/poll.h

Include poll.h instead of sys/poll.h

poll.h is defined in POSIX:
http://pubs.opengroup.org/onlinepubs/009695399/functions/poll.html

This fixes a compile warning when building with musl libc:
In file included from start.c:46:0:
/usr/include/sys/poll.h:1:2: error: #warning redirecting incorrect #include <sys
/poll.h> to <poll.h> [-Werror=cpp]
 #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
  ^

Signed-off-by: Natanael Copa <ncopa@alpinelinux.org>
Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com>