patch 8.0.0069

Problem:    Compiler warning for self-comparison.
Solution:   Define ONE_WINDOW and add #ifdef.

src/buffer.c

@@ -4935,7 +4935,7 @@ do_arg_all(
 		    }
 #ifdef FEAT_WINDOWS
 		    /* don't close last window */
-		    if (firstwin == lastwin
+		    if (ONE_WINDOW
 			    && (first_tabpage->tp_next == NULL || !had_tab))
 #endif
 			use_firstwin = TRUE;