avtools: move media_type_string() from avprobe to cmdutils.

It will be useful outside of avprobe.

cmdutils.h

@@ -514,6 +514,11 @@ FILE *get_preset_file(char *filename, size_t filename_size,
  */
 void *grow_array(void *array, int elem_size, int *size, int new_size);
 
+/**
+ * Get a string describing a media type.
+ */
+const char *media_type_string(enum AVMediaType media_type);
+
 #define GROW_ARRAY(array, nb_elems)\
     array = grow_array(array, sizeof(*array), &nb_elems, nb_elems + 1)