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: NEric Blake <eblake@redhat.com>

.gnulib @ 3f51bf41

-Subproject commit d5fec6c22f03c6a73d62260c9ce091c10c0a9cbd
+Subproject commit 3f51bf41c8be8b310f57caff371377414701d5cc