maint.mk: avoid echo -e

maint.mk: avoid echo -e

There are still other potentially non-portable uses of echo
throughout this file, if $(ME) contains \ or starts with -,
but this patch is a trivial step in the right direction.

* top/maint.mk (gzip_rsyncable): Convert all uses of echo -* to
printf.
Reported by Matthias Bolte.

Signed-off-by: Eric Blake <eblake@redhat.com>