daemon: exclude requirement for probes.h on systems without systemtap

This 1-liner was actually written by Eric Blake, over IRC. It
addresses a compilation failure in make dist and make rpm for
systems without the dtrace/systemtap development libraries
installed.

daemon/Makefile.am

@@ -168,7 +168,7 @@ EXTRA_DIST += probes.d libvirtd.stp
 
 if WITH_DTRACE
 libvirtd_LDADD += probes.o
-libvirtd_SOURCES += probes.h
+libvirtd_nodist_SOURCES = probes.h
 
 BUILT_SOURCES += probes.h