build: update gnulib for canonicalize_filename_mode improvements

This is required for the new realpath command.

diff --git a/gnulib b/gnulib
index f5c2e2ac7d4ca2f6ba15e56a245f348899360a00..943bf5a92b5a7a3e4b3a7178654c5491244ee8aa 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit f5c2e2ac7d4ca2f6ba15e56a245f348899360a00
+Subproject commit 943bf5a92b5a7a3e4b3a7178654c5491244ee8aa