build: update to latest gnulib

* gnulib: Update to pick up support for dd.mm.yy format
in parse-datetime.

diff --git a/gnulib b/gnulib
index 83730551b451b43577531986a5c4300e0166c42f..6a978b7d2febb63a6b538a1d3f428607c49a8ae4 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 83730551b451b43577531986a5c4300e0166c42f
+Subproject commit 6a978b7d2febb63a6b538a1d3f428607c49a8ae4