dsputil.h is not a system header, use "" when #including it.

Originally committed as revision 13092 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/pngenc.c

@@ -20,8 +20,8 @@
  */
 #include "avcodec.h"
 #include "bytestream.h"
+#include "dsputil.h"
 #include "png.h"
-#include <dsputil.h>
 
 /* TODO:
  * - add 2, 4 and 16 bit depth support