change Include to only use *.conf files

A little background on this:

Currently I use an homegrown collectd module which is OK, but I want to replace it with the pdxcat one for various reasons. I need to distribute a custom types DB file with additional types, which is placed as custom.types.db in the conf.d directory and a custom.types.conf file with
```
TypesDB "/etc/collectd/conf.d/custom.types.db"
```
in it. If all files in conf.d are included, this doesn't work, so I included only *.conf files.