ipv6: ila: select CONFIG_DST_CACHE

My randconfig builds came across an old missing dependency for ILA:

ERROR: "dst_cache_set_ip6" [net/ipv6/ila/ila.ko] undefined!
ERROR: "dst_cache_get" [net/ipv6/ila/ila.ko] undefined!
ERROR: "dst_cache_init" [net/ipv6/ila/ila.ko] undefined!
ERROR: "dst_cache_destroy" [net/ipv6/ila/ila.ko] undefined!

We almost never run into this by accident because randconfig builds
end up selecting DST_CACHE from some other tunnel protocol, and this
one appears to be the only one missing the explicit 'select'.

>From all I can tell, this problem first appeared in linux-4.9
when dst_cache support got added to ILA.

Fixes: 79ff2fc31e0f ("ila: Cache a route to translated address")
Cc: Tom Herbert <[e-mail]>
Signed-off-by: Arnd Bergmann <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/Kconfig

···
 config IPV6_ILA
 	tristate "IPv6: Identifier Locator Addressing (ILA)"
 	depends on NETFILTER
+	select DST_CACHE
 	select LWTUNNEL
 	---help---
 	  Support for IPv6 Identifier Locator Addressing (ILA).