mpegvideo: convert mpegvideo_common.h to a .c file

mpegvideo: convert mpegvideo_common.h to a .c file

This file defines a single, huge function, MPV_motion(), which
although being declared inline is not actually inlined by the
compiler (for good reason).  There is thus no sense in defining
this function in a header file, resulting in multiple copies of
it in the final library.

Signed-off-by: Mans Rullgard <mans@mansr.com>