exim: security bump to 4.87.1

No features are added or removed. This release contains
just a fix for CVE-2016-9963

    - Fix CVE-2016-9963 - Info leak from DKIM.  When signing DKIM, if
      either LMTP or PRDR was used for delivery, the key could appear in
      logs.  Additionally, if the experimental feature "DSN_INFO" was used,
      it could appear in DSN messages (and be sent offsite).

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>