iso15118: Announce iso15118 feature. Fixes #503.

- No hardware detection: Each charger with this module is assumed to
  have the PLC modem
- Not checking is_enabled(): The feature announces that the hardware
  is available, not that the charger is configured to use it.

Author: rtrbt

software/src/modules/iso15118/iso15118.cpp

@@ -390,6 +390,8 @@ void ISO15118::setup()
 
     iso15118.trace("ISO15118: EVSEID ISO: %s, DIN: %s", evseid_iso, din_str);
 
+    api.addFeature("iso15118");
+
     initialized = true;
 
     if (is_enabled()) {