gst/: Use GLib versions of htonl, htons, ntohl and ntohs in order to avoid problems on win32 (#529707).

Original commit message from CVS:
* gst/rtp/gstasteriskh263.c: (gst_asteriskh263_chain):
* gst/udp/gstmultiudpsink.c: (gst_multiudpsink_add_internal):
* gst/udp/gstudpsrc.c: (gst_udpsrc_start):
Use GLib versions of htonl, htons, ntohl and ntohs in order
to avoid problems on win32 (#529707).