lavu/timecode: Increase AV_TIMECODE_STR_SIZE.

Fixes the following warning:
libavutil/timecode.c:103:60: warning: '%02d' directive output may be truncated writing between 2 and 10 bytes into a region of size between 0 and 7

libavutil/timecode.h

@@ -30,7 +30,7 @@
 #include <stdint.h>
 #include "rational.h"
 
-#define AV_TIMECODE_STR_SIZE 16
+#define AV_TIMECODE_STR_SIZE 23
 
 enum AVTimecodeFlag {
     AV_TIMECODE_FLAG_DROPFRAME      = 1<<0, ///< timecode is drop frame