fix regression: meta_workspace_get_neighbor does not work
with multiple workspace rows.

The regression appears to have been introduced here:
622599d557429a3fe1a0babd6c2cb4abb7f64f3e  "Allow cycling through workspaces if the gsettings key is set"
Merge pull request #69 from autarkper/workspace-neighbor-fix

fix regression: meta_workspace_get_neighbor does not work with multiple workspace rows.