build: update gnulib submodule to latest

Includes a change to xalloc.h to avoid -Wstrict-overflow warnings
with GCC 5.1 on 32 bit with optimization enabled.  A subsequent
commit will fix similar issues in shred.

diff --git a/gnulib b/gnulib
index 6439a0af0ca6b0256a4273a8ae0175e896bcc8da..213d5545f67d726ccffa13842577be47aca93d8d 160000 (submodule)
--- a/gnulib
+++ b/gnulib
@@ -1 +1 @@
-Subproject commit 6439a0af0ca6b0256a4273a8ae0175e896bcc8da
+Subproject commit 213d5545f67d726ccffa13842577be47aca93d8d