collectd: fix riemann write plugin dependencies

collectd: fix riemann write plugin dependencies

Fixes:
http://autobuild.buildroot.org/results/fe5/fe5b5ed6355a794e84894c4aaf62eda6529ed184/
http://autobuild.buildroot.org/results/6c3/6c393cffb6ad4e676e311e9fc23ddbb2bcc2cf36/

The plugin uses the riemann-c-client library since commit d55584214206
(write_riemann: Use riemann-c-client), so adjust the dependencies to match.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>