Allow tmpfs in ksysguard disk space monitor

As tmpfs mounts are more widely used for shared memory in browsers like Chromium and Firefox, there is a need for more and more space in such filesystems, with grave circumstances when the space is exhausted - it causes instability, crashes or black display in both of these browsers. Given this importance, I suggest tmpfs should not be blacklisted from disk space reporting in ksysguardd. Because tmpfs "devices" don't start with a /, it is needed to obviate the test for "real" device names as well.

Note: this is an old patch from phabricator https://phabricator.kde.org/D23704 (hasn't been accepted though)