mkv.cpp: avoid gcc warnings

modules/demux/mkv.cpp

@@ -269,7 +269,7 @@ typedef struct
 {
     vlc_bool_t   b_default;
     vlc_bool_t   b_enabled;
-    int         i_number;
+    unsigned int i_number;
 
     int          i_extra_data;
     uint8_t      *p_extra_data;
@@ -535,7 +535,7 @@ public:
     EbmlStream         *p_es;
 
     std::vector<matroska_segment_t*> segments;
-    int                              i_current_segment;
+    size_t                           i_current_segment;
 
     demux_sys_t                      & sys;