maint: add gnulib's update-copyright module

* bootstrap.conf (gnulib_modules): Add update-copyright, so
that each year we can run "make update-copyright".
Otherwise, that rule fails due to the absence of the file
in build-aux.

diff --git a/bootstrap.conf b/bootstrap.conf
line changes: +1/-0
index 9d9c5e4..6d33b1e
--- a/bootstrap.conf
+++ b/bootstrap.conf

@@ -37,6 +37,7 @@ gnulib_modules="
     non-recursive-gnulib-prefix-hack
     progname
     readme-release
+    update-copyright
     wchar
 "