client/server: fix ptr <-> uint64_t casting warnings on 32-bit builds

Not actually a bug, but gcc complains.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/server.c b/server.c
index db9d859..ad07a0e 100644
--- a/server.c
+++ b/server.c
@@ -299,7 +299,7 @@
 
 	cmd = malloc(sizeof(*cmd));
 
-	fio_init_net_cmd(&cmd->cmd, opcode, NULL, 0, (uint64_t) cmd);
+	fio_init_net_cmd(&cmd->cmd, opcode, NULL, 0, (uintptr_t) cmd);
 	fio_net_cmd_crc(&cmd->cmd);
 
 	INIT_FLIST_HEAD(&cmd->list);