net: ipv6: select DST_CACHE from IPV6_RPL_LWTUNNEL

The rpl sr tunnel code contains calls to dst_cache_*() which are
only present when the dst cache is built.
Select DST_CACHE to build the dst cache, similar to other kconfig
options in the same file.
Compiling the rpl sr tunnel without DST_CACHE will lead to linker
errors.

Fixes: a7a29f9c361f ("net: ipv6: add rpl sr tunnel")
Signed-off-by: Thomas Weißschuh <[e-mail]>
Reviewed-by: Simon Horman <[e-mail]>
Tested-by: Simon Horman <[e-mail]> # build-tested
Signed-off-by: David S. Miller <[e-mail]>

net/ipv6/Kconfig

···
 	bool "IPv6: RPL Source Routing Header support"
 	depends on IPV6
 	select LWTUNNEL
+	select DST_CACHE
 	help
 	  Support for RFC6554 RPL Source Routing Header using the lightweight
 	  tunnels mechanism.