Avoid compiler warnings for taglib headers

Summary:
Declaring them as system headers (-> -isystem) tells the compiler
we cannot do anything about them, so it stays silent for them.

Completes b680861aa2ed973eb6694a9dda1c327107323062

Test Plan:
Still builds and installs, but no longer warns about issues in taglib
headers.

Reviewers: mgallien

Reviewed By: mgallien

Subscribers: kde-frameworks-devel, Baloo

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D13914