cksum: don't leak memory using -a sha3 with OpenSSL

* gnulib: Update to the latest commit for latest changes to the
crypto/sha3-buffer and crypto/sha3 modules.

diff --git a/gnulib b/gnulib
index 06e7da510bd055f524c1baff0890c861bef2b6a4..68ecc26f30b284a26ab9aa0a08299869ea713c71 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 06e7da510bd055f524c1baff0890c861bef2b6a4
+Subproject commit 68ecc26f30b284a26ab9aa0a08299869ea713c71