Don't include <fstab.h> anymore

Summary:
It was replaced with Solid in
f835801a.
Furthermore, musl currently doesn't provide it (might get implemented
in the future [1]) and filelight thus fails to build against musl.

[1] https://www.openwall.com/lists/musl/2018/03/25/1

Test Plan: Builds fine against musl and glibc

Reviewers: sandsmark

Subscribers: kde-utils-devel

Differential Revision: https://phabricator.kde.org/D18780