New QtMultimedia dependency for --enable-qt6/--enable-kf6 builds (by default)

[Michael Weghorn]

In order to make video playback work for the Qt 6 based VCL plugins (qt6, kf6) work, the pending Gerrit change [1] ("tdf#145735 avmedia qt: Use QtMultimedia for Qt 6 media playback") will introduce a new default dependency on QtMultimedia for builds using the --enable-qt6/--enable-kf6 autogen options, except for WASM.

[1] https://gerrit.libreoffice.org/c/core/+/167869

If you're building these, please make sure the corresponding development package (e.g. qt6-multimedia-dev on Debian) is installed or explicitly disable the use of QtMultimedia using the new autogen option "--disable-qt6-multimedia" once the change has landed on master.

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