KQueue.hsc: fix build failure on FreeBSD

Build failed as:
  libraries/base/GHC/Event/KQueue.hsc:192:25: error:
      Not in scope: type constructor or class ‘Int16’
      |
  192 | newtype Filter = Filter Int16
      |                         ^^^^^

If was caused by an import tweak from Word16 to Int16.
Adjust imports to make KQueue compile cleanly.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

Test Plan: build on freebsd

Reviewers: bgamari, austin, hvr

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3300