Fill UDS_CREATION_TIME with the value of st_birthtime on FreeBSD

Summary:
It seems UDS_CREATION_TIME is never filled at all (as far as I could find in the short time I invested :D ).

From the manpage:

st_birthtim      Time when the inode was created.

This allows us to display file creation dates in (patched) dolphin.

This is with respect to:
https://bugs.kde.org/show_bug.cgi?id=374063

Reviewers: rakuco, dfaure, arrowdodger, tcberner

Reviewed By: dfaure, arrowdodger

Subscribers: aacid, kfunk, emmanuelp, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D5138