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

Tested-by: Anisse Astier <anisse@astier.eu>

Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/ipv4/Kconfig b/net/ipv4/Kconfig
index aa2a2c7..d183262 100644
--- a/net/ipv4/Kconfig
+++ b/net/ipv4/Kconfig
@@ -409,7 +409,7 @@
 
 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.