Linux softraid: Define _GNU_SOURCE for pipe2

Summary:
In a strict POSIX compilation environment, pipe2 is not defined.  Use the _GNU_SOURCE FTM to expose it.

Fixes a failure to build ksysguard on Adélie Linux.

Reviewers: #plasma, davidedmundson, broulik

Reviewed By: #plasma, davidedmundson

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D19865