maint: update to latest gnulib

Among others, this fixes getgroups for MacOS and fpending for
DragonFly BSD.

* .gnulib: Update to latest.

Signed-off-by: Eric Blake <eblake@redhat.com>

diff --git a/.gnulib b/.gnulib
index 8f7425866463f994538584d1dd7211603b8b0550..831b84c59ef413c57a36b67344467d66a8a2ba70 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit 8f7425866463f994538584d1dd7211603b8b0550
+Subproject commit 831b84c59ef413c57a36b67344467d66a8a2ba70