The qcow2 driver needs the zlib dependency.  While emulators
provided it through the migration code, this is not true of
the tools.  Move the dependency from the qcow1 rule directly
into block_ss so that it is included unconditionally.

Fixes build with --disable-qcow1.

Reported-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>