build-sys: extract rootfs for make check

Commit "d2c2b8b build-sys: autotoolify build of check libraries" removed
rootfs extraction. Add it back.
diff --git a/Makefile.am b/Makefile.am
index 22e882b..2dc09a3 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -133,6 +133,10 @@
 # TESTSUITE
 # ------------------------------------------------------------------------------
 
+testsuite/rootfs:
+	$(AM_V_GEN) tar -C testsuite/ \
+		-xJf $(top_srcdir)/testsuite/rootfs.tar.xz
+
 TESTSUITE_OVERRIDE_LIBS = testsuite/uname.la testsuite/path.la \
 			  testsuite/init_module.la \
 			  testsuite/delete_module.la