Lack of "start" member function declare in WBThrottle.h

Fix urgent mistake.
see http://gitbuilder.sepia.ceph.com/gitbuilder-ceph-deb-precise-amd64-gcov/log.cgi?log=b8fb366eab2c7fff27732087f250b0eb1ab7dd79

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>