Fix configure with Qt5Multimedia disabled.

Summary:
When passed -DBUILD_TESTING=OFF -DCMAKE_DISABLE_FIND_PACKAGE_Qt5Multimedia=ON,
CMake failed because find_package(Qt5 COMPONENTS MULTIMEDIA) was treated as
required due to the previous find_package(Qt5 REQUIRED COMPONENTS Xml) call.

Finding Qt5Multimedia directly via its own config file ensures that it really
is treated as optional.

Test Plan: Configures and builds both with and without Qt5Multimedia enabled.

Reviewers: Frameworks, mgallien

Reviewed By: mgallien

Subscribers: mgallien, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6366