examples/gnrc_border_router: request temp address

Additionally to requesting a delegated prefix via DHCPv6 this also
enables to ask for a temporary (non-permanent) address on the 6lbr. If
the upstream DHCP server provides one, this can be used to establish
end-to-end connectivity from the 6lbr towards an Internet host directly.

examples/gnrc_border_router/Makefile

@@ -57,6 +57,7 @@ endif
 ifeq (dhcpv6,$(PREFIX_CONF))
   # include DHCPv6 client for 6LoWPAN border router
   USEMODULE += gnrc_dhcpv6_client_6lbr
+  USEMODULE += dhcpv6_client_ia_na
 else ifeq (uhcp,$(PREFIX_CONF))
   # include UHCP client
   USEMODULE += gnrc_uhcpc