Simply enabling the eap module like so:
```
    freeradius::module::eap { 'eap':
        ensure => 'present',
    }
```
results in a broken configuration:
```
...
including configuration file /etc/raddb/mods-enabled/eap
/etc/raddb/mods-enabled/eap[604]: Too many closing braces
Errors reading or parsing /etc/raddb/radiusd.conf
```

Move the opening `gtc {` outside the block that is conditional on the
definition of `@gtc_challenge`