Fix out of tree build with hardcoded tables

Originally committed as revision 21228 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mdct_tablegen.h

@@ -35,7 +35,7 @@ SINETABLE(1024);
 SINETABLE(2048);
 SINETABLE(4096);
 #else
-#include "mdct_tables.h"
+#include "libavcodec/mdct_tables.h"
 #endif
 
 SINETABLE_CONST float * const ff_sine_windows[] = {