D12674: Mark `Phonon4Qt5` dependency as optional in CMakeLists file

[Alexander Schlarb]

ntninja added a reviewer: kde-frameworks-devel.

REVISION DETAIL
  https://phabricator.kde.org/D12674

To: ntninja, kde-frameworks-devel
Cc: kde-frameworks-devel, apol, ltoscano, cgiboudeaux, michaelh, ngraham, bruns