tools: Link libvirt.la and readline to libvirt_shell.a

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>