build: update to latest gnulib

Pick up recent build fixes to avoid sysctl.h inclusion on glibc systems,
restrict the max file size supported by read-file to PTRDIFF_MAX,
and to avoid a -Werror=unused failure in test-canonicalize.

diff --git a/gnulib b/gnulib
index 7d069378921bfa0d7c7198ea177aac0a2440016f..203dda3b86d5c9ac5a661d61038c284370448ca8 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 7d069378921bfa0d7c7198ea177aac0a2440016f
+Subproject commit 203dda3b86d5c9ac5a661d61038c284370448ca8