rxrpc: Fix incorrect conditional on IPV6

The udpv6_encap_enable() function is part of the ipv6 code, and if that is
configured as a loadable module and rxrpc is built in then a build failure
will occur because the conditional check is wrong:

net/rxrpc/local_object.o: In function `rxrpc_lookup_local':
local_object.c:(.text+0x2688): undefined reference to `udpv6_encap_enable'

Use the correct config symbol (CONFIG_AF_RXRPC_IPV6) in the conditional
check rather than CONFIG_IPV6 as that will do the right thing.

Fixes: 5271953cad31 ("rxrpc: Use the UDP encap_rcv hook")
Reported-by: [e-mail]
Reported-by: Arnd Bergmann <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Reviewed-by: Arnd Bergmann <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/rxrpc/local_object.c

···
 	udp_sk(usk)->gro_complete = NULL;
 
 	udp_encap_enable();
-#if IS_ENABLED(CONFIG_IPV6)
+#if IS_ENABLED(CONFIG_AF_RXRPC_IPV6)
 	if (local->srx.transport.family == AF_INET6)
 		udpv6_encap_enable();
 #endif