pldm: Include config.h via compiler argument

This removes any ambiguity about which config.h is being included.
Subprojects such as libpldm and phosphor-logging also generate config.h
files, and these are exposed in the include path.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Change-Id: I3e0ccd6339f088380ffa29d41167b07aefc7dd16