Make DST_CACHE a silent config option

commit 911362c7

 ("net: add dst_cache support") added a new
kconfig option that gets selected by other networking options.
It seems the intent wasn't to offer this as a user-selectable
option given the lack of help text, so this patch converts it
to a silent option.

Signed-off-by: Dave Jones <davej@codemonkey.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>