cmake: fix externalproject for latest host-ipmid

phosphor-host-ipmid did a conversion to meson but our externalproject
implementation was not updated correspondingly.  Fix that up now and
do a minor header path change necessary per the latest upstream code.

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: I8cfe67a61d14e71b4cad217e13cac2030b707bd3
2 files changed