build: update gnulib submodule to latest

The previous gnulib submodule reference was *still* to a
non-public commit.  My submodule had a stray commit, so
the reference was always to a local merge commit.
Reported by Rob Vermaas.

diff --git a/gnulib b/gnulib
index 680c7ff..ff4bb04 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 680c7ff005a2eb96b0903e5200cc28de273555c7
+Subproject commit ff4bb04bec0c8d7834bfb0590d6de08fa0ceefd0