maint: gnulib: upgrade.

diff --git a/gnulib b/gnulib
index 44c4f75581a7774594e5a225ab43c9c211e4abb4..50bb21eab7dfc87bbfcbc75f0232407110cdd296 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 44c4f75581a7774594e5a225ab43c9c211e4abb4
+Subproject commit 50bb21eab7dfc87bbfcbc75f0232407110cdd296