Move libffi to pkgconfig Requires.private
    
    Currently most of the apps that link with libgirepository are also
    linked with libffi, without actually using any symbols from there.
    Moving libffi to Requires.private avoids needlessly linking with libffi
    and should reduce the impact when the libffi soname bump lands in
    rawhide.
    
    Backported from upstream commit a847eb8.