debian: bump compat to 9

debian: bump compat to 9

dh_auto_configure set -DCMAKE_BUILD_TYPE=None so the default does not
take effect at all. this is on purpose, see [1].

and dpkg-buildflags is able to produce the suggested compiling flags by
debian policy. dh_* can pass these flags to cmake only if it works at
compat>=9, see [2].

and we are safe to move to compat 9, as jewel's supported debian based
distro is jessie (with debhelper 9.20150101), ubuntu trusty (with
debhelper 9.20131227ubuntu1)

---
[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=701233#35
[2] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=653916

Fixes: http://tracker.ceph.com/issues/16744
Signed-off-by: Kefu Chai <kchai@redhat.com>