build: Fix build with dtrace + apparmor

Building virt-aa-helper with dtrace probes enabled, ldd complained about
undefined references:

./.libs/libvirt_util.a(libvirt_util_la-event_poll.o):(.note.stapsdt+0x24):
undefined reference to `libvirt_event_poll_purge_timeout_semaphore'
...

src/Makefile.am

@@ -1569,6 +1569,9 @@ virt_aa_helper_LDADD =						\
 		libvirt_conf.la					\
 		libvirt_util.la					\
 		../gnulib/lib/libgnu.la
+if WITH_DTRACE_PROBES
+virt_aa_helper_LDADD += probes.o
+endif
 virt_aa_helper_CFLAGS =						\
 		-I$(top_srcdir)/src/conf			\
 		-I$(top_srcdir)/src/security			\