Bugfix: missing use cvmix_get_put in convection

- A missing use statement led to compile-time errors at the link
  stage.
- Problem was introduced in 8a02a4c756d5021dca576810bd64bd4162cd21a3.
Merge pull request #28 from CVMix/bugfix/convection_use_get_put

Bugfix/convection use get put