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

net/Kconfig

@@ -397,7 +397,7 @@ config LWTUNNEL
 	  with light weight tunnel state associated with fib routes.
 
 config DST_CACHE
-	bool "dst cache"
+	bool
 	default n
 
 config NET_DEVLINK