sysutils/rsnapshot: unbreak with Perl 5.18

• Backport an upstreamed patch which unbreaks rsnapshot with Perl >= 5.18.
• Bump PORTREVISION

PR: 195446
Differential Revision: https://reviews.freebsd.org/D1324
Submitted by: David O'Rourke <dor.bsd@fastmail.fm>
Approved by: remko (maintainer)
Approved by: koobs (mentor)