amt: add IPV6 Kconfig dependency

This driver cannot be built-in if IPV6 is a loadable module:

x86_64-linux-ld: drivers/net/amt.o: in function `amt_build_mld_gq':
amt.c:(.text+0x2e7d): undefined reference to `ipv6_dev_get_saddr'

Add the idiomatic Kconfig dependency that all such modules
have.

Fixes: b9022b53 ("amt: add control plane of amt interface")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Taehee Yoo <ap420073@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/Kconfig

@@ -294,6 +294,7 @@ config GTP
 config AMT
 	tristate "Automatic Multicast Tunneling (AMT)"
 	depends on INET && IP_MULTICAST
+	depends on IPV6 || !IPV6
 	select NET_UDP_TUNNEL
 	help
 	  This allows one to create AMT(Automatic Multicast Tunneling)