Declare the url_write buffer parameter as const

libavformat/avio.c

libavformat/avio.h

libavformat/file.c

libavformat/gopher.c

libavformat/http.c

libavformat/librtmp.c

libavformat/rtmpproto.c

libavformat/rtpproto.c

libavformat/tcp.c

libavformat/udp.c