build: Include correct poll.h
Including sys/poll.h when poll.h is also present produces a compile warning on some systems, but only sys/poll.h is present on other systems, such as AIX. This makes sure the most suitable poll.h is included in each situation.
This again mainly functions to silence a compile error, but there is a possibility that it may fix building on some system that only has poll.h rather than sys/poll.h, although I have yet to become aware of such a case.
Thank you for your time.