build: use a gnulib SHA1 that is not private

* gnulib: Update submodule to latest *public*.

diff --git a/gnulib b/gnulib
index f4cec507097f7d494af250b8a2ddc76aa885c86c..c90c5cc82c3b0c07b2dff0f135be74bbb97e0b60 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit f4cec507097f7d494af250b8a2ddc76aa885c86c
+Subproject commit c90c5cc82c3b0c07b2dff0f135be74bbb97e0b60