* .gitignore: add sdpmessage autogenerated files to ignore list
  * gstreamer/src/filelist.am: add sdpmessage to a build
  * gstreamer/src/sdpmessage.{ccg|hg}: add wrapper for GstSDPKey
    and GstSDPMedia classes.
  * tools/m4/convert_gst.m4: conversion for sdpmessage enum types