purple: 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 +134,15 @@ impl_extra_handler_builder! {
     get_chat_name => GetChatNameHandler
     send_im => SendIMHandler
     chat_send => ChatSendHandler
-    get_cb_alias => GetChatBuddyAlias
+}
+
+// get_cb_alias is only available in Purple 2.14
+#[cfg(libpurple2_14)]
+impl<T: crate::purple::handlers::traits::GetChatBuddyAlias> RegisterContext<T> {
+    #[allow(dead_code)]
+    pub fn enable_get_cb_alias(mut self) -> Self {
+        self.extra_info.get_cb_alias =
+            Some(crate::purple::handlers::entrypoints::get_cb_alias::<T>);
+        self
+    }
 }