syms: add forgotten virAdmConnectClass symbol

Although it currently doesn't cause any linking issues, the symbol should be
exported correctly according to our conventions.

src/libvirt_admin_private.syms

@@ -12,6 +12,7 @@ xdr_admin_connect_list_servers_ret;
 xdr_admin_connect_open_args;
 
 # datatypes.h
+virAdmConnectClass;
 virAdmGetServer;
 virAdmServerClass;