package/at: fix parallel build failure

Add a patch to finally fix parallel build failure. Patch is pending
upstream:
https://salsa.debian.org/debian/at/merge_requests/14



Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>