maint: update to latest gnulib

Among other patches, Natanael Copa contributed a fix for
pyshmem_available when using musl libc that directly affects
libvirt.

* .gnulib: Update to latest, in part for musl libc compat.
Signed-off-by: NEric Blake <eblake@redhat.com>

.gnulib @ e8e0eb6b

-Subproject commit ce457d622d947d8aa3e325d98a2818d097e7fec9
+Subproject commit e8e0eb6bfb728685ec8d5afd924e41b18e9d928d