community/libwpebackend-fdo: put .so symlink in main package

the .so file here is not strictly for development but rather it's directly used by other things (and other distributions handle it accordingly, see e.g. https://packages.ubuntu.com/hirsute/amd64/libwpebackend-fdo-1.0-1/filelist)

this is a preparatory change for introduction of WPE Webkit and Cog