net: Fix build regression when INET_UDP_DIAG=y and IPV6=m

Tested-by: NAnisse Astier <anisse@astier.eu>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/Kconfig

@@ -409,7 +409,7 @@ config INET_TCP_DIAG
 
 config INET_UDP_DIAG
 	tristate "UDP: socket monitoring interface"
-	depends on INET_DIAG
+	depends on INET_DIAG && (IPV6 || IPV6=n)
 	default n
 	---help---
 	  Support for UDP socket monitoring interface used by the ss tool.