qtmultimedia: don't autolink all available backends in static builds

So while working on updating Qt in vcpkg (https://github.com/microsoft/vcpkg/pull/26693)
and fixing static builds of qtmultimedia with gstreamer/ffmpeg backends enabled
I have observed that qtinterfaceframework fails to build due to linker errors due to duplicated symbols from the windows/gstreamer/ffmpeg backends. So I assume that the CMake scripts of Qt just try to autolink all those backends together which of course fails. qtmultimedia should fix this by deactivating autolinking for static builds and just link one preferred backend,