Have dtrace depend on object files directly, not objfiles.txt

Per Mark Johnston, this resolves a build error on FreeBSD related
to the fact that dtrace is modifying the generated object files
under the hood.  Consequently, without this, dtrace gets reinvoked
at install time because the object files have been updated.  This
is a pretty hacky fix, but it shouldn't hurt anything, and it's
not clear that it's worth expending any more effort for a feature
that not too many people are using.

Patch by Mark Johnston.  This is arguably back-patchable as a bug
fix to the build system, but I'm not certain enough of the
consequences to try that.  Let's see what the buildfarm (and
our packagers) think of this change on master first.

src/backend/Makefile

@@ -188,8 +188,8 @@ $(top_builddir)/src/include/utils/probes.h: utils/probes.h
 	    $(LN_S) "../../../$(subdir)/utils/probes.h" .
 
 
-utils/probes.o: utils/probes.d $(SUBDIROBJS)
-	$(DTRACE) $(DTRACEFLAGS) -C -G -s $(call expand_subsys,$^) -o $@
+utils/probes.o: utils/probes.d $(call expand_subsys,$(SUBDIROBJS))
+	$(DTRACE) $(DTRACEFLAGS) -C -G -s $< $(call expand_subsys,$(SUBDIROBJS)) -o $@
 
 
 ##########################################################################