Add sample crc64 checksum

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/crc64.h b/crc64.h
new file mode 100644
index 0000000..e4310c2
--- /dev/null
+++ b/crc64.h
@@ -0,0 +1,6 @@
+#ifndef CRC64_H
+#define CRC64_H
+
+unsigned long long crc64(const unsigned char *, unsigned long);
+
+#endif