build: update to latest gnulib, for syntax-check

This picks up a fix for a syntax-check weakness mentioned here:
https://www.redhat.com/archives/libvir-list/2013-May/msg00811.html

* .gnulib: Update to latest, for maint.mk improvement.
Signed-off-by: NEric Blake <eblake@redhat.com>

.gnulib @ a363f4ed

-Subproject commit 0f8ddbf1a13a93a4584afdd82f0f87e498a0db6f
+Subproject commit a363f4ed4a0e69187c97686ac44502c49c7f4b3d