iwyu/mkvmuxerutil: Update includes.

Include what you use analysis based include update.

Change-Id: I6cd977b2bbeee4a8ef960a074aed52db35db4d89

mkvmuxerutil.cpp

@@ -20,6 +20,7 @@
 #include <ctime>
 #include <new>
 
+#include "mkvmuxer.hpp"
 #include "mkvwriter.hpp"
 #include "webmids.hpp"
 

mkvmuxerutil.hpp

@@ -9,11 +9,11 @@
 #ifndef MKVMUXERUTIL_HPP
 #define MKVMUXERUTIL_HPP
 
-#include "mkvmuxer.hpp"
 #include "mkvmuxertypes.hpp"
 
 namespace mkvmuxer {
-
+class Cluster;
+class Frame;
 class IMkvWriter;
 
 const uint64 kEbmlUnknownValue = 0x01FFFFFFFFFFFFFFULL;