Fix compilation error in case unix_fd is used in async_method_call
Fix compilation error in case unix_fd is used in async_method_call

unix_fd has fd sanitization in its append routine. It makes it impossible
to use unix_fd in async_method_call, as it explicitly applies const qualifier
to all message arguments.

To keep previous behavior in current unix_fd clients I've added specialization
inside append routine for variables passed by const reference.

Signed-off-by: Adrian Ambrożewicz <5d596128d78c438d49333a98311ba7a5bf287c3e@linux.intel.com>
Change-Id: I3409f9f2b40d14d9486eca203321333ad1bb2efa

openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus,openbmc/sdbusplus