Fix aiocb compile warnings on HPUX

Unlike other platforms, it seems to require aiocb64 explicitly
for long offsets. Add an os independent typedef for this.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/ioengine.h b/ioengine.h
index 75c2c1a..044c4da 100644
--- a/ioengine.h
+++ b/ioengine.h
@@ -22,7 +22,7 @@
 		struct iocb iocb;
 #endif
 #ifdef FIO_HAVE_POSIXAIO
-		struct aiocb aiocb;
+		os_aiocb_t aiocb;
 #endif
 #ifdef FIO_HAVE_SGIO
 		struct sg_io_hdr hdr;