poll.h is mandated by Single Unix Spec v2, the usual baseline for
postgres on unix.  None of the unixoid buildfarms animals has
sys/poll.h but not poll.h.  Therefore there's not much point to test
for sys/poll.h's existence and include it optionally.

Author: Andres Freund, per suggestion from Tom Lane
Discussion: https://postgr.es/m/20505.1492723662@sss.pgh.pa.us