restore_pipe_data(): fix a typo

Found using clang:

cr-restore.c:796:8: error: use of unary operator that may be intended as compound assignment (+=)
                size =+ ret;
                     ^~
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>

cr-restore.c

@@ -789,7 +789,7 @@ static int restore_pipe_data(struct pipe_entry *e, int wfd, int pipes_fd)
 			return -1;
 		}
 
-		size =+ ret;
+		size += ret;
 	}
 
 	return 0;