libpldmresponder: Make libpldmresponder optional

libpldmresponder library is for handling incoming PLDM request messages
on BMC. There are usecases for PLDM daemon on BMC where it is primarily
a requester and BMC aggregates information about firmware devices that
implement MCTP/PLDM and facilitate firmware upgrade of the devices.
This patch is a step in that direction to enable PLDM daemon on BMC
primarily as PLDM requester.

Tested: Ran PLDM daemon successfully without libpldmresponder library.

Signed-off-by: Tom Joseph <rushtotom@gmail.com>
Change-Id: Iceaa10127352aaaba6c338d7ea9ac187be4e5fad