udp_diag: Make it module when ipv6 is a module

Eric Dumazet reported, that when inet_diag is built-in the udp_diag also goes
built-in and when ipv6 is a module the udp6 lookup symbol is not found.

  LD      .tmp_vmlinux1
net/built-in.o: In function `udp_dump_one':
udp_diag.c:(.text+0xa2b40): undefined reference to `__udp6_lib_lookup'
make: *** [.tmp_vmlinux1] Erreur 1

Fix this by making udp diag build mode depend on both -- inet diag and ipv6.
Reported-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NPavel Emelyanov <xemul@parallels.com>
Acked-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/Kconfig

@@ -411,7 +411,7 @@ config INET_TCP_DIAG
 
 config INET_UDP_DIAG
 	depends on INET_DIAG
-	def_tristate INET_DIAG
+	def_tristate INET_DIAG && IPV6
 
 menuconfig TCP_CONG_ADVANCED
 	bool "TCP: advanced congestion control"