redfish.hpp: fix header path

This must be historical reason that we use relative path in these
includes.

Given that redfish-core/lib is already in include path, these relative
paths are not necessary.

Tested: code compiles.

Signed-off-by: Nan Zhou <nanzhoumails@gmail.com>
Change-Id: Ia6b6be8a7aded98c96cdca150467bc7d825230af