Don't import fcntl on Windows

In 9b73c19b2e04e6db0dc70046e0ba5e5e22446996 the fcntl import was
mistakenly added to the general imports instead of the platform-
specific ones.  This fixes the problem so lockutils works on
Windows again.

Closes-Bug: #1335897

Change-Id: Iadcc6562faeacbe887c3dcafe06d54bd08caea07