Fix glib_subprocess test

A typo means that the tests dependent on glib with subprocess
support are never run.

Fixes: 9d41401b

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>