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: 5271953c

 ("rxrpc: Use the UDP encap_rcv hook")
Reported-by:  <kbuild-all@01.org>
Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>