Fix compilation with emscripten when ENABLE_WEBMTS is enabled

Change-Id: I8d4af8de67bb340ead880c2205bccef6736507e4
diff --git a/common/video_frame.cc b/common/video_frame.cc
index bfebf7a..00aa8da 100644
--- a/common/video_frame.cc
+++ b/common/video_frame.cc
@@ -7,6 +7,8 @@
 // be found in the AUTHORS file in the root of the source tree.
 #include "common/video_frame.h"
 
+#include <cstdio>
+
 namespace libwebm {
 
 bool VideoFrame::Buffer::Init(std::size_t new_length) {