Fix C compatibility in header.

This struct is later used without the struct keyword, so it's not
valid C.

Test: treehugger
Bug: https://github.com/chadversary/android-ndk-rutabagas/blob/f3b3cd3de6b49f33ad82c1318c2ee280e5231c8d/src/main/ru_media.c#L41
Change-Id: I6ecd0614815749c0c17b8327bde6c8035fbd9932
diff --git a/media/ndk/include/media/NdkMediaCodec.h b/media/ndk/include/media/NdkMediaCodec.h
index 8fb6a87..80d5d50 100644
--- a/media/ndk/include/media/NdkMediaCodec.h
+++ b/media/ndk/include/media/NdkMediaCodec.h
@@ -114,12 +114,12 @@
         int32_t actionCode,
         const char *detail);
 
-struct AMediaCodecOnAsyncNotifyCallback {
+typedef struct AMediaCodecOnAsyncNotifyCallback {
       AMediaCodecOnAsyncInputAvailable  onAsyncInputAvailable;
       AMediaCodecOnAsyncOutputAvailable onAsyncOutputAvailable;
       AMediaCodecOnAsyncFormatChanged   onAsyncFormatChanged;
       AMediaCodecOnAsyncError           onAsyncError;
-};
+} AMediaCodecOnAsyncNotifyCallback;
 
 #if __ANDROID_API__ >= 21