LU-12226 debian: fix debhelper level requirement

LU-12226 debian: fix debhelper level requirement

Requiring debhelper >= 11 will break the build on
Ubuntu 16.04 LTS, it only has debhelper 9 and 10
from backports.

As we dropped the build for 16.04, change the required
debhelper to 11, which is the version shipping with
18.04

Change-Id: I40838a4de5ee7a841ba970d4f7531f8fe9dc5ff4
Signed-off-by: Li Dongyang <dongyangli@ddn.com>
Reviewed-on: https://review.whamcloud.com/34765
Reviewed-by: Minh Diep <mdiep@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>