patch 9.1.0825: compile error for non-diff builds

Problem:  compile error for non-diff builds
          (after v9.1.0822)
Solution: Add #ifdef FEAT_DIFF (John Marriott)

Signed-off-by: John Marriott <basilisk@internode.on.net>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/evalfunc.c b/src/evalfunc.c
index f6a3f9d..adae129 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -8359,15 +8359,19 @@
 	{
 	    if (switch_win_noblock(&switchwin, wp, tp, TRUE) == OK)
 	    {
+#ifdef FEAT_DIFF
 		// in diff mode, prevent that the window scrolls
 		// and keep the topline
 		if (curwin->w_p_diff && switchwin.sw_curwin->w_p_diff)
 		    skip_update_topline = TRUE;
+#endif
 		check_cursor();
 		fp = var2fpos(&argvars[0], TRUE, &fnum, FALSE);
 	    }
+#ifdef FEAT_DIFF
 	    if (curwin->w_p_diff && switchwin.sw_curwin->w_p_diff)
 		skip_update_topline = FALSE;
+#endif
 	    restore_win_noblock(&switchwin, TRUE);
 	}
     }