maint: update to latest gnulib

Among other things, gnulib now allows overriding of sed during
'make syntax-check'.

* .gnulib: Update to latest, for maint.mk improvements.

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

diff --git a/.gnulib b/.gnulib
index d5fec6c22f03c6a73d62260c9ce091c10c0a9cbd..3f51bf41c8be8b310f57caff371377414701d5cc 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit d5fec6c22f03c6a73d62260c9ce091c10c0a9cbd
+Subproject commit 3f51bf41c8be8b310f57caff371377414701d5cc