For IReferenceClock.

Currently we rely on it getting included via the following path:

wmsdk.idl
 -> wmsdkidl.idl
    -> drmexternals.idl
       -> wmdrmsdk.idl
          -> mfobjects.idl
             -> mediaobj.idl
                -> strmif.idl

However, mediaobj.idl should not pull in strmif.idl.