THRIFT-5666: lib: cpp: prefer poll.h over sys/poll.h

The `<sys/poll.h>` header is not actually where `poll(2)` must be
defined according to the spec. However, it may be the case that
some random UNIX-like OS uses `<sys/poll.h>` instead.

Both use cases can be supported simply by including the proper
header, if it exists.

Signed-off-by: Chris Friedt <cfriedt@meta.com>
4 files changed