Xpoll.h.in: use standard poll.h include
avoids a bunch of warnings when building against musl libc that warns on this pedantically:
In file included from /usr/include/X11/Xpoll.h:166:
/usr/include/sys/poll.h:1:2: warning: redirecting incorrect #include <sys/poll.h> to <poll.h> [-W#warnings]