vp10: add extern "C" to headers

Change-Id: Ie2e8b37fa01ce8d6b993684f431f3159d511cfb1

vp10/decoder/dthread.h

@@ -15,6 +15,10 @@
 #include "vpx_util/vpx_thread.h"
 #include "vpx/internal/vpx_codec_internal.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct VP10Common;
 struct VP10Decoder;
 
@@ -63,4 +67,8 @@ void vp10_frameworker_broadcast(RefCntBuffer *const buf, int row);
 void vp10_frameworker_copy_context(VPxWorker *const dst_worker,
                                   VPxWorker *const src_worker);
 
+#ifdef __cplusplus
+}    // extern "C"
+#endif
+
 #endif  // VP10_DECODER_DTHREAD_H_