Prefer idx_t to size_t in xheader.c

Prefer idx_t to size_t in xheader.c

* src/xheader.c (x_obstack_grow, x_obstack_blank)
(xheader_format_name, xheader_ghdr_name, xheader_write)
(struct xhdr_tab, locate_handler, decode_record, decx, decg)
(xheader_read, xattr_encode_keyword, xheader_print_n)
(xheader_string_end, dummy_decoder, atime_decoder, gid_decoder)
(gname_decoder, linkpath_decoder, ctime_decoder, mtime_decoder)
(path_decoder, sparse_path_decoder, size_decoder, uid_decoder)
(uname_decoder, sparse_size_decoder, sparse_numblocks_decoder)
(sparse_offset_coder, sparse_offset_decoder)
(sparse_numbytes_coder, sparse_numbytes_decoder)
(sparse_map_decoder, dumpdir_decoder, volume_label_decoder)
(volume_size_decoder, volume_offset_decoder)
(volume_filename_decoder, xattr_selinux_decoder)
(xattr_acls_a_decoder, xattr_acls_d_decoder, xattr_coder)
(xattr_decoder, sparse_major_decoder, sparse_minor_decoder):
Prefer idx_t to size_t.