ghostpad: Do nothing in _internal_activate_push_default

ghostpad: Do nothing in _internal_activate_push_default

When calling gst_pad_activate_mode() on a ghostpad
gst_ghost_pad_activate_push_default() will be called. This will call
gst_pad_activate_mode() on the proxypad (which is internal of the
ghostpad), calling gst_ghost_pad_internal_activate_push_default(), which
again will call gst_pad_activate_mode() on the original ghostpad.

By simply returning TRUE in
gst_ghost_pad_internal_activate_push_default() the redundant call to
gst_pad_activate_mode() (for the same pad) is avoided.

https://bugzilla.gnome.org/show_bug.cgi?id=761913