Adding API to get vpx encoder/decoder interface.

Change-Id: I137e5e6585356792913e1e84da6c0a439c5153a5
diff --git a/vpxenc.h b/vpxenc.h
index 5103ee6..1e6acaa 100644
--- a/vpxenc.h
+++ b/vpxenc.h
@@ -22,9 +22,11 @@
   TEST_DECODE_WARN,
 };
 
+struct VpxInterface;
+
 /* Configuration elements common to all streams. */
 struct VpxEncoderConfig {
-  const struct codec_item *codec;
+  const struct VpxInterface *codec;
   int passes;
   int pass;
   int usage;