top-level: add extern "C" to headers

Change-Id: I5164df72aff84eca0ace56032c5373f04053c6a5
diff --git a/webmdec.h b/webmdec.h
index 002fbe6..fa5a52e 100644
--- a/webmdec.h
+++ b/webmdec.h
@@ -12,6 +12,10 @@
 
 #include "./tools_common.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 struct nestegg;
 struct nestegg_packet;
 struct VpxInputContext;
@@ -37,4 +41,8 @@
 
 void webm_free(struct WebmInputContext *webm_ctx);
 
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
 #endif  // WEBMDEC_H_