Add support for the Google xxhash checksumming function

It's a lot faster than the other software defined ones, so will
be a useful alternative for the cases where crc32c-intel isn't
available.

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/fio.1 b/fio.1
index 4377052..81b03f7 100644
--- a/fio.1
+++ b/fio.1
@@ -985,7 +985,7 @@
 .RS
 .RS
 .TP
-.B md5 crc16 crc32 crc32c crc32c-intel crc64 crc7 sha256 sha512 sha1
+.B md5 crc16 crc32 crc32c crc32c-intel crc64 crc7 sha256 sha512 sha1 xxhash
 Store appropriate checksum in the header of each block. crc32c-intel is
 hardware accelerated SSE4.2 driven, falls back to regular crc32c if
 not supported by the system.