Add missing backslash to update-gnulib.sh

A user on irc noticed a missing backslash on one line in
update-gnulib.sh.  This patch adds it.

Re-running update-gnulib.sh causes a few copyright notices to change.
Presumably these are from upstream gnulib and shouldn't be touched by
our yearly update.  I've updated the script to account for this, but I
did not want to try testing it...