Commit 34e8f63a altered virfile.o to drag in additional symbols,
which in turn led to pulling in other .o files and eventually causing
a link failure when systemtap probes are enabled, such as:

./.libs/libvirt_util.a(libvirt_util_la-event_poll.o): In function `virEventPollRunOnce':
/home/dummy/libvirt/src/util/event_poll.c:614: undefined reference to `libvirt_event_poll_run_semaphore'
./.libs/libvirt_util.a(libvirt_util_la-event_poll.o):(.note.stapsdt+0x24): undefined reference to `libvirt_event_poll_add_handle_semaphore'

Even though libvirt_iohelper and libvirt_parthelper don't directly
use the portion of virfile.o that drags in probing, it was easier
to satisfy the linker and get the build back up, than to figure out
whether it is even possible or worth trying to disentangle the mess.

* src/Makefile.am (libvirt_iohelper_LDADD)
(libvirt_parthelper_LDADD): Use libvirt_probes.lo when needed.