Include the 2 byte length field for the optional "extra data"

field when computing the length of the gzip header.

Thanks to Dag-Erling for pointing me to the OpenSSH tarballs,
which are the first files I've seen that actually used this field.

lib/libarchive/archive_read_support_compression_gzip.c

@@ -148,6 +148,7 @@ peek_at_header(struct archive_read_filter *filter, int *pbits)
 		if (p == NULL)
 			return (0);
 		len += ((int)p[len + 1] << 8) | (int)p[len];
+		len += 2;
 	}
 
 	/* Null-terminated optional filename. */