ipmid-api: wrap in extern "C"
ipmid-api: wrap in extern "C"

Wrap the entire header in an ifdef'd extern "C" so it can be used from C
files.  This fixes compilation for c files that need the header.

Signed-off-by: Joel Stanley <4816dabf8db1bc6cac35b3a24cab2ff844b5b0c7@jms.id.au>

openbmc/phosphor-host-ipmid,shgoupf/phosphor-host-ipmid,vishwabmc/phosphor-host-ipmid,causten/phosphor-host-ipmid,openbmc/phosphor-host-ipmid,shgoupf/phosphor-host-ipmid,vishwabmc/phosphor-host-ipmid,causten/phosphor-host-ipmid,causten/phosphor-host-ipmid,openbmc/phosphor-host-ipmid