patch 8.0.1121: can uncheck executables in MS-Windows installer

Problem:    Can uncheck executables in MS-Windows installer.
Solution:   Make the choice read-only. (Ken Takata, closes #2106)
diff --git a/nsis/gvim.nsi b/nsis/gvim.nsi
index 8bfe5e6..1a169f3 100644
--- a/nsis/gvim.nsi
+++ b/nsis/gvim.nsi
@@ -179,7 +179,7 @@
 
 ##########################################################
 Section "Vim executables and runtime files"
-	SectionIn 1 2 3
+	SectionIn 1 2 3 RO
 
 	# we need also this here if the user changes the instdir
 	StrCpy $0 "$INSTDIR\vim${VER_MAJOR}${VER_MINOR}"