block/raw-posix: Make hdev_aio_discard() available outside Linux

Fixes the build on OpenBSD among others.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Andreas Färber <andreas.faerber@web.de>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>