build-sys: declare rootfs.tar.xz as dep of rootfs/

This forces make to re-extract the tarball when it's changed
diff --git a/Makefile.am b/Makefile.am
index 04c0c64..5395019 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -133,7 +133,7 @@
 # TESTSUITE
 # ------------------------------------------------------------------------------
 
-testsuite/rootfs:
+testsuite/rootfs: $(top_srcdir)/testsuite/rootfs.tar.xz
 	$(AM_V_GEN) tar -C testsuite/ \
 		-xJf $(top_srcdir)/testsuite/rootfs.tar.xz