build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index 4aa11814c9e9979b82e07c8793723d50ac183920..6b7a999681243e3aed9089a7697cf51119de04b1 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 4aa11814c9e9979b82e07c8793723d50ac183920
+Subproject commit 6b7a999681243e3aed9089a7697cf51119de04b1