packages: ceph.spec.in is missing make as a build dependency

On a virgin centos-6.4, after yum-builddep ceph and following
http://ceph.com/docs/next/install/building-ceph/ instructions to:

cd ceph
./autogen.sh
./configure
make

it fails because make is not installed. It probably is not a problem for
most people because there are few developers who did not install make.

Signed-off-by: Loic Dachary <loic@dachary.org>