Make the function prototype visible to comply with C99 inline.

Fixes building with gcc -std=gnu99.

Originally committed as revision 14140 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/i386/idct_sse2_xvid.c

@@ -39,6 +39,7 @@
  */
 
 #include "libavcodec/dsputil.h"
+#include "libavcodec/i386/idct_xvid.h"
 
 /*!
  * @file idct_sse2_xvid.c