build: update gnulib submodule to latest

To provide argmatch_exact() that does not
use abbreviated matching, to be used by cksum.

diff --git a/gnulib b/gnulib
index a3e1e74e65af0649224d84813df2e9ebea2e9a67..6ef3d783333346333f35bb181ba90f5bc46c0b29 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit a3e1e74e65af0649224d84813df2e9ebea2e9a67
+Subproject commit 6ef3d783333346333f35bb181ba90f5bc46c0b29