As for most ebuild failing with the new headers, its caused by the removal of linux/videodev.h. media-video/motion-3.2.12 (which is already in the tree) works fine!
With "media-video/motion-3.2.12 works fine" I mean it compiles fines, although it silently drops the v4l use flag and disables the v4l-support.
*** This bug has been marked as a duplicate of bug 361509 ***