(struct xhdr_tab.coder): Last arg is void const *,

(struct xhdr_tab.coder): Last arg is void const *,
not void *, to avoid create.c warning from GCC "passing argument 3
of 'xheader_store' discards qualifiers from pointer target type".
(dummy_coder, atime_coder, gid_coder, gname_coder, linkpath_coder):
(ctime_coder, mtime_coder, path_coder, size_coder, uid_coder):
(uname_coder, sparse_size_coder, sparse_numblocks_coder):
(sparse_offset_coder, sparse_numbytes_coder, dumpdir_coder):
Likewise.