tools: Link libvirt.la and readline to libvirt_shell.a
    
    So, our mingw build is broken. It's because while libvirt_shell
    library is using some of our internal APIs, e.g. virStrndup, and
    readline API but it's not being linked with nor libvirt.la nor
    libreadline.  Only subsequent users of the library, like virsh,
    do link to the needed libraries. In fact, I'm surprised Linux
    linker doesn't care, because how can it make a static library
    with missing symbols is mystery to me.
    
    Signed-off-by: Michal Privoznik <mprivozn@redhat.com>