ipmi: fix build config variable name for ipmi_bmc_extern.o

ipmi: fix build config variable name for ipmi_bmc_extern.o

The original commit:

  commit 67aa56fc03bea44ccf384ea400515a8a58844a50
  Author: Corey Minyard <cminyard@mvista.com>
  Date:   Thu Dec 17 12:50:06 2015 -0600

    ipmi: Add an external connection simulation interface

defined a new variable CONFIG_IPMI_EXTERN, but then went
on to mistakely use the pre-existing CONFIG_IPMI_LOCAL
variable.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Corey Minyard <cminyard@mvista.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>