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: a7a29f9c ("net: ipv6: add rpl sr tunnel")
Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Simon Horman <horms@kernel.org> # build-tested
Signed-off-by: David S. Miller <davem@davemloft.net>