Mark libcls_kvs as a module

This was missed by the changes in commit 909a733fe7119fc1bf123b3739e3acc6f2135b6a

Signed-off-by: James Page <james.page@ubuntu.com>
Merge pull request #876 from javacruft/wip-missed-module

Mark libcls_kvs as a module

Reviewed-by: Sage Weil <sage@inktank.com>