build: update gnulib submodule to latest

diff --git a/gnulib b/gnulib
index 123f3ec7335c8c15b613f234fb65453c31e4667f..ac2fdcad666d24d51a32a152fe1b3911b610344d 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 123f3ec7335c8c15b613f234fb65453c31e4667f
+Subproject commit ac2fdcad666d24d51a32a152fe1b3911b610344d