build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index 2905392e0cff45e852e211e1d5eb7cfc5cf98a7f..15cd8edb6ec9aed2585e10456d46eec09d5c1b8b 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 2905392e0cff45e852e211e1d5eb7cfc5cf98a7f
+Subproject commit 15cd8edb6ec9aed2585e10456d46eec09d5c1b8b