Enable parallel build support

Thanks: Felix Geyer for pointing this out

debian/rules

@@ -111,7 +111,7 @@ LOGROTATE = $(basename $(basename $(notdir $(wildcard daemon/libvirtd*.logrotate
 EXAMPLES_DIR = $(CURDIR)/debian/libvirt-doc/usr/share/doc/libvirt-doc/examples/
 
 %:
-	dh $@ --builddirectory=$(DEB_BUILDDIR)
+	dh $@ --builddirectory=$(DEB_BUILDDIR) --parallel
 
 override_dh_auto_configure:
 	dh_auto_configure -- $(DEB_CONFIGURE_EXTRA_ARGS)