Add Alexander Moisseev to contributors for sysutils/backuppc4

He is also the maintainer of these ports:
mail/roundcube-contextmenu
mail/roundcube-thunderbird_labels
net/rsync-bpc
sysutils/backuppc
sysutils/backuppc-devel
sysutils/p5-BackupPC-XS
www/free-sa-devel

PR: 219020
Approved by: garga (mentor)
Differential Revision: https://reviews.freebsd.org/D10979