ext/wavpack/gstwavpackparse.c: Use G_GINT64_CONSTANT, this fixes the duration query on files without known length.

Original commit message from CVS:
* ext/wavpack/gstwavpackparse.c: (gst_wavpack_parse_reset),
(gst_wavpack_parse_src_query), (gst_wavpack_parse_create_src_pad):
Use G_GINT64_CONSTANT, this fixes the duration query on files without
known length.