Makefile.am:  fix ceph_sbindir

This reinstates the fix for the ceph_sbindir from commit
352f362567bf270d0896fb7573df4ae5139a56fb, with corrections
from Danny's review commits pull request #389.
Fixes: #5492

Reported-by: Denis Kaganovich <mahatma@eu.by>
Reviewed-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>
Signed-off-by: Sage Weil <sage@inktank.com>
Signed-off-by: Gary Lowell  <gary.lowell@inktank.com>