mkv: fix declaration/definition mismatch of demux_sys_t

Kill many compiler warnings.

modules/demux/mkv/chapter_command.hpp

@@ -35,7 +35,7 @@ const binary MATROSKA_DVD_LEVEL_PG   = 0x18;
 const binary MATROSKA_DVD_LEVEL_PTT  = 0x10;
 const binary MATROSKA_DVD_LEVEL_CN   = 0x08;
 
-class demux_sys_t;
+struct demux_sys_t;
 
 class chapter_codec_cmds_c
 {

modules/demux/mkv/demux.hpp

@@ -326,7 +326,7 @@ private:
 };
 
 
-class demux_sys_t
+struct demux_sys_t
 {
 public:
     demux_sys_t( demux_t & demux )

modules/demux/mkv/mkv.cpp

@@ -73,7 +73,7 @@ vlc_module_begin ()
     add_shortcut( "mka", "mkv" )
 vlc_module_end ()
 
-class demux_sys_t;
+struct demux_sys_t;
 
 static int  Demux  ( demux_t * );
 static int  Control( demux_t *, int, va_list );