Update plugins.qmltypes for QtMultimedia

The file contains meta information about the QML plugin which permits QML code completion for the QtMultimedia QML types. It can be generated via "make qmltypes" in the relevant qml plugin dir.

This must be done for every release similar to writing the changes file.

The update was not done for Qt 5.4 and it should be redone for Qt 5.5 again as each of those releases has introduced new QML types.