top-level: add extern "C" to headers

Change-Id: I5164df72aff84eca0ace56032c5373f04053c6a5
diff --git a/webmenc.h b/webmenc.h
index f3bc3ec..362aa89 100644
--- a/webmenc.h
+++ b/webmenc.h
@@ -23,6 +23,10 @@
 #include "tools_common.h"
 #include "vpx/vpx_encoder.h"
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 typedef off_t EbmlLoc;
 
 struct cue_entry {
@@ -84,4 +88,8 @@
 
 void write_webm_file_footer(struct EbmlGlobal *glob, int hash);
 
+#ifdef __cplusplus
+}  // extern "C"
+#endif
+
 #endif  // WEBMENC_H_