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

[Michael Weghorn <m.weghorn@xxxxxxxxx>]

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.

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.

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

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