Add avcore.h header in samplefmt.h.

Avoid the need to manually include the avcore.h header, less
error-prone.

Originally committed as revision 26333 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcore/samplefmt.h

@@ -19,6 +19,8 @@
 #ifndef AVCORE_SAMPLEFMT_H
 #define AVCORE_SAMPLEFMT_H
 
+#include "avcore.h"
+
 /**
  * all in native-endian format
  */