build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index bc33a8a0c77285b2bdb5c5d0a4243f7b442a0293..68f693ff1db33bf24695f0f42c62e7801966fd06 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit bc33a8a0c77285b2bdb5c5d0a4243f7b442a0293
+Subproject commit 68f693ff1db33bf24695f0f42c62e7801966fd06