mail/exim: build breaks if EMBEDDED_PERL is unset

• build depends on perl5 present, for exigrep

PR: 275023
Reported-by: Andrew "RhodiumToad" Gierth <andrew@tao11.riddles.org.uk>
Submitted-by: zirias
Approved-by: fluffy (implicit)
Author: Felix Palmen <zirias@FreeBSD.org>