add apt-get install pkg-config for ubuntu server

Signed-off-by: Kai Zhang <kaizh.pub@gmail.com>
Merge pull request #929 from kazhang/add-pkg-config

add apt-get install pkg-config for ubuntu server

Reviewed-by: Loic Dachary <loic@dachary.org>
Reviewed-by: Sage Weil <sage@inktank.com>