gst/silence/gstsilence.*: Add sync property.

gst/silence/gstsilence.*: Add sync property.

Original commit message from CVS:
* gst/silence/gstsilence.c: (gst_silence_class_init),
(gst_silence_set_clock), (gst_silence_get),
(gst_silence_set_property), (gst_silence_get_property):
* gst/silence/gstsilence.h: Add sync property.
* gst/sine/gstsinesrc.c: (gst_sinesrc_class_init),
(gst_sinesrc_init), (gst_sinesrc_set_clock), (gst_sinesrc_get),
(gst_sinesrc_set_property), (gst_sinesrc_get_property):
* gst/sine/gstsinesrc.h: Add sync property.