mail/sendmail: fix stage-qa error

The strip command for sbin/sendmail fails due to the permissions of
the program installed in the stage directory. Change the staged sendmail
mode to 755 so that the strip command succeeds. The pkg-list specifies
the final mode to be 2555.

PR: 211353
Reported by: dave (ci.com.au)
Final fix: marino@
Appproved by: maintainer timeout