gnulib: update git submodule

Signed-off-by: Daiki Ueno <ueno@gnu.org>

diff --git a/gnulib b/gnulib
index 3639c57a970191e0bf7a9789bd1341786d0255a1..6ec9bc73686fd42d0a1a82c914d89176745cddbd 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 3639c57a970191e0bf7a9789bd1341786d0255a1
+Subproject commit 6ec9bc73686fd42d0a1a82c914d89176745cddbd