Move createUDSEntry from file.cpp to file_unix.cpp

Summary:
This was a unix only function in the shared file protocol cpp
This has not been used at all in file_win.cpp for a while

Reviewers: Frameworks, dfaure

Reviewed By: dfaure

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25301