Fix ‘make dist-xz’ bug

Problem reported by Pavel Raiskup in:
https://lists.gnu.org/r/bug-tar/2022-06/msg00014.html
* bootstrap.conf (copy_files): Also copy DISTFILES.

bootstrap.conf

@@ -79,7 +79,7 @@ gnulib_modules="$avoided_gnulib_modules
 
 # copy_files srcdir dstdir
 copy_files() {
-  for file in `cat $1/DISTFILES`
+  for file in DISTFILES `cat $1/DISTFILES`
   do
     case $file in
     "#*")  continue;;