build: update to latest gnulib

In particular, this is a quick gnulib fix for a mingw compile error.

* .gnulib: Update to latest.
Signed-off-by: NEric Blake <eblake@redhat.com>

.gnulib @ c3b13129

-Subproject commit 5ddd9d713d58d6c6d10494a28ceee6988d20ff98
+Subproject commit c3b131294aa42b7997cc9b9a0bbb2934aa27fd6c