Missing WiFiClass::setHostname definition

[dansiviter]

The value is defined in the header file, but it has no definition resulting in:

undefined reference to `arduino::WiFiClass::setHostname(char const*)'

Related: #863

[Channel59]

This can be implemented rather easily if the patches in #921 are applied, similarly how it is done for the Ethernet class.

[pennam]

release 4.2.1 adds dhcp hostname feature