libs/libplacebo: Fix deploying with androiddeployqt
This library is used by Tokodon, by proxy through MpvQt (and underneath, MPV and so on.) However, Meson tries to append the soversion to the end of the library filename, which androiddeployqt rejects. We have to work around that by setting the SONAME explicitly, and then moving the versioned .so file so it's only one.