(tar_checksum): New function

src/common.h

@@ -501,6 +501,7 @@ void print_for_mkdir (char *, int, mode_t);
 void print_header (struct tar_stat_info *, off_t);
 void read_and (void (*) (void));
 enum read_header read_header (bool);
+enum read_header tar_checksum (union block *header);
 void skip_file (off_t);
 void skip_member (void);