patch 8.1.0228: dropping files is ignored while Vim is busy

Problem:    Dropping files is ignored while Vim is busy.
Solution:   Postpone the effect of dropping files until it's safe.
diff --git a/src/gui.h b/src/gui.h
index 13c97e6..9ac5e07 100644
--- a/src/gui.h
+++ b/src/gui.h
@@ -65,8 +65,9 @@
 /*
  * GUIs that support dropping files on a running Vim.
  */
-#if defined(FEAT_GUI_MSWIN) || defined(FEAT_GUI_MAC) \
-	|| defined(FEAT_GUI_GTK)
+#if (defined(FEAT_DND) && defined(FEAT_GUI_GTK)) \
+	|| defined(FEAT_GUI_MSWIN) \
+	|| defined(FEAT_GUI_MAC)
 # define HAVE_DROP_FILE
 #endif