Support libpurple 2.13

get_cb_alias was introduced in 2.14 and referencing it directly in this
macro breaks compilation against libpurple-2.13, as found in Debian
Buster.

https://docs.pidgin.im/pidgin/2.x.y/struct__PurplePluginProtocolInfo.html#ad43cef005f1634b0d170cc2972c69550

src/purple/loader.rs

@@ -134,5 +134,4 @@ impl_extra_handler_builder! {
     get_chat_name => GetChatNameHandler
     send_im => SendIMHandler
     chat_send => ChatSendHandler
-//    get_cb_alias => GetChatBuddyAlias
 }