rpm: use updated gperftools

rpm: use updated gperftools

make sure we only build with the higher version of gperftools on
distros where both 2.4 and 2.6.1 are packaged. see
https://git.centos.org/summary/rpms!gperftools.git . at the time of
writing, gperftools 2.6.1 is packaged for CentOS/RHEL 7, if gperftools
(>= 2.4) is required by Ceph, and user already has this version
installed, when new Ceph packages are installed, the updated gperftools
2.6.1 version won't be installed as a dependency. when launching
Ceph compiled with tcmalloc enabled, we will have

symbol lookup error: ceph-osd: undefined symbol: _ZdaPvm

so, by bumping up the required version of gperftools, the updated
gperftools will be installed.

see https://software.opensuse.org/package/gperftools, openSUSE/SLE offer
2.5. so they are safe at this moment.

Fixes: http://tracker.ceph.com/issues/35969
Signed-off-by: Kefu Chai <kchai@redhat.com>