rxrpc: Fix missing IPV6 #ifdef

Fix rxrpc_encap_err_rcv() to make the call to ipv6_icmp_error conditional
on IPV6 support being enabled.

Fixes: b6c66c43 ("rxrpc: Use the core ICMP/ICMP6 parsers")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
cc: netdev@vger.kernel.org