ext/wavpack/gstwavpackparse.*: Always report the duration if we know it in push mode and don't return 0 just to make ...

Original commit message from CVS:
* ext/wavpack/gstwavpackparse.c: (gst_wavpack_parse_src_query),
(gst_wavpack_parse_create_src_pad):
* ext/wavpack/gstwavpackparse.h:
Always report the duration if we know it in push mode and don't
return 0 just to make totem believe we can't seek in push mode.
Newer totem version use the SEEKING query which properly reports
if we can seek or not.