maint: update to latest gnulib

Future patches need LGPLv2+ versions of some modules that had
recent license changes; but separating the gnulib update from
the actual use of the modules makes it easier to backport to
an older version while avoiding a submodule update (assuming,
of course, that the backport is to a system where glibc provides
adequate functionaliy without needing the gnulib module).

* .gnulib: Update to latest, for modules needed in later patches.
Signed-off-by: NEric Blake <eblake@redhat.com>
(cherry picked from commit 7961ad21)

.gnulib @ da8d59ee

-Subproject commit b72ff2a45efde544c406804186d37a3254728571
+Subproject commit da8d59ee79138b85daee1ad5b22ea12e4fb6ce47