build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index 881b91b9b370b4b6f26eb44efb5c048ca3783b83..6f6420cc9705dcfa545a28c674fddf5703e72c86 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 881b91b9b370b4b6f26eb44efb5c048ca3783b83
+Subproject commit 6f6420cc9705dcfa545a28c674fddf5703e72c86