collectd: Adjust .json names to match plugin names

collectd: Adjust .json names to match plugin names

Adjust .json names to match the plugin names: use '-' instead of '_'
Fix commits 033c19a and e36b727 that used a filename differing
from the plugin name used in OpenWrt. That prevented package compilation
to find and include the .json files that were left out of the package.

Signed-off-by: Hannu Nyman <redacted>