Do not #include common.h without necessity.

This eliminates the need to work around common.h overriding certain system
functions which are required in this header.

Originally committed as revision 16776 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavutil/mem.c

@@ -25,12 +25,6 @@
  */
 
 #include "config.h"
-#include "common.h"
-
-/* here we can use OS dependent allocation functions */
-#undef malloc
-#undef free
-#undef realloc
 
 #include <limits.h>
 #include <stdlib.h>