build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index 387fd77e70fe9016d30e462d46c1126b32fe449b..b20e8afb0b2cc9548fac073d71fcd19ede29ce60 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 387fd77e70fe9016d30e462d46c1126b32fe449b
+Subproject commit b20e8afb0b2cc9548fac073d71fcd19ede29ce60