Fix build without Phonon

Summary:
f6d55baf5aa88eaab6b2f96c025064f081d13cea replaced ${PHONON_LIBS} with
Phonon's imported target. This breaks in the case when Phonon isn't
found or disabled via -DCMAKE_DISABLE_FIND_PACKAGES_Phonon4Qt5=TRUE
because the imported target isn't known. It worked previously because
${PHONON_LIBS} was just empty when Phonon wasn't available.

Test Plan:
Building without phonon works now and and it still builds
with phonon available and successfully links to it.

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D17479