patch 8.0.0660: silent install on MS-Windows shows dialog

Problem:    Silent install on MS-Windows does show a dialog.
Solution:   Add /SD to the default choice. (allburov, closes #1772)
diff --git a/nsis/gvim.nsi b/nsis/gvim.nsi
index 74a5433..9ace4ea 100644
--- a/nsis/gvim.nsi
+++ b/nsis/gvim.nsi
@@ -87,7 +87,7 @@
 Function .onInit
   MessageBox MB_YESNO|MB_ICONQUESTION \
 	"This will install Vim ${VER_MAJOR}.${VER_MINOR} on your computer.$\n Continue?" \
-	IDYES NoAbort
+	/SD IDYES NoAbort
 	    Abort ; causes installer to quit.
 	NoAbort: