gst-libs/gst/sdp/gstsdpmessage.*: Constify args where we can.

gst-libs/gst/sdp/gstsdpmessage.*: Constify args where we can.

Original commit message from CVS:
* gst-libs/gst/sdp/gstsdpmessage.c: (gst_sdp_media_get_media),
(gst_sdp_media_get_port), (gst_sdp_media_get_num_ports),
(gst_sdp_media_get_proto), (gst_sdp_media_formats_len),
(gst_sdp_media_get_format), (gst_sdp_media_get_information),
(gst_sdp_media_connections_len), (gst_sdp_media_get_connection),
(gst_sdp_media_bandwidths_len), (gst_sdp_media_get_badwidth),
(gst_sdp_media_get_key), (gst_sdp_media_attributes_len),
(gst_sdp_media_get_attribute), (gst_sdp_media_get_attribute_val_n),
(gst_sdp_media_get_attribute_val):
* gst-libs/gst/sdp/gstsdpmessage.h:
Constify args where we can.