Fix build on FreeBSD broken by d7cce9937d5e9af2753fadb82d11f308b58bb8fa

Summary: Support hidden NTFS files only on OS that support extended file attributes. For this purpose check if sys/xattr.h exist via CMake.

Reviewers: Dolphin, kossebau, dfaure

Reviewed By: dfaure

Subscribers: ngraham, bcooksley, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11716