Makefile: fix ceph_sbindir

From Denis Kaganovich <mahatma@eu.by>:

At least in some cases, scripts (ceph-disk*, ceph-create-keys)
installing into /usr/usr/sbin. This happened while
"ceph_sbindir = $(prefix)$(sbindir)" in src/Makefile.am and "
./configure --prefix=/usr ..." (Gentoo's default configure call).
I don't dig too deep, but IMHO it can be result of "prefix=" &
"exec_prefix=" disorder. Fixed for me by "ceph_sbindir = $(sbindir)"
(I don't try other configure behaviors, but looks universal).

Reported-by: Denis Kaganovich <mahatma@eu.by>
Fixes: #5492
Signed-off-by: Sage Weil <sage@inktank.com>