[IPV4]: Fix thinko in TCP_CONG_BIC default.

Since it is tristate when we offer it as a choice, we should
definte it also as tristate when forcing it as the default.
Otherwise kconfig warns.
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/Kconfig

@@ -549,7 +549,7 @@ config TCP_CONG_SCALABLE
 endmenu
 
 config TCP_CONG_BIC
-	boolean
+	tristate
 	depends on !TCP_CONG_ADVANCED
 	default y