Re-add a dependency in the make-bootstrap-tarball target

- Whoops, the dependency on the tarball got removed when fixing the
dependencies around the tarball file. Re-add that so the rootfs
continuous build will work.

Change-Id: I22c5d0913ca87ef7b20edc3c7f38e797320c46e1
diff --git a/debootstrap.mk b/debootstrap.mk
index 4897f44..b5b88a6 100644
--- a/debootstrap.mk
+++ b/debootstrap.mk
@@ -27,6 +27,8 @@
 	+make -f $(ROOTDIR)/build/debootstrap.mk make-bootstrap-sha256sum
 endif
 
+make-bootstrap-tarball: $(DEBOOTSTRAP_TARBALL)
+
 make-bootstrap-sha256sum: $(DEBOOTSTRAP_TARBALL)
 	cd $(ROOTDIR)/cache && \
 		sha256sum $(notdir $(DEBOOTSTRAP_TARBALL)) > $(DEBOOTSTRAP_TARBALL_SHA256)