iwyu/sample_muxer_metadata: Update includes.

Include what you use analysis based include update.

Change-Id: Ib3fa6a5a3b6afadf56d52f7feb9264e18b454cd3

sample_muxer_metadata.cc

@@ -10,7 +10,11 @@
 // library, which allows clients to handle a matroska format file.
 
 #include "sample_muxer_metadata.h"
+
+#include <cstdio>
 #include <string>
+
+#include "mkvmuxer.hpp"
 #include "vttreader.h"
 
 SampleMuxerMetadata::SampleMuxerMetadata() : segment_(NULL) {}

sample_muxer_metadata.h

@@ -11,10 +11,18 @@
 
 #include <list>
 #include <set>
+#include <string>
 
-#include "mkvmuxer.hpp"
+#include "mkvmuxertypes.hpp"
 #include "webvttparser.h"
 
+namespace mkvmuxer {
+class Chapter;
+class Frame;
+class Segment;
+class Track;
+}  // namespace mkvmuxer
+
 class SampleMuxerMetadata {
  public:
   enum Kind { kSubtitles, kCaptions, kDescriptions, kMetadata, kChapters };