libxvid: add some const fixing a warning

Originally committed as revision 24138 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/libxvid_internal.h

@@ -27,6 +27,6 @@
  */
 
 
-int ff_tempfile(char *prefix, char **filename);
+int ff_tempfile(const char *prefix, char **filename);
 
 #endif /* AVCODEC_LIBXVID_INTERNAL_H */

libavcodec/libxvidff.c

@@ -85,7 +85,7 @@ void xvid_correct_framerate(AVCodecContext *avctx);
  * *prefix can be a character constant; *filename will be allocated internally.
  * @return file descriptor of opened file (or -1 on error)
  * and opened file name in **filename. */
-int ff_tempfile(char *prefix, char **filename) {
+int ff_tempfile(const char *prefix, char **filename) {
     int fd=-1;
 #if !HAVE_MKSTEMP
     *filename = tempnam(".", prefix);