[LyX/master] Disable assigment operator

[Georg Baum]

commit 15a5e0f7936650328ea28ef38988cdbb92f62a27
Author: Georg Baum <b...@lyx.org>
Date:   Sun Oct 11 12:04:43 2015 +0200

    Disable assigment operator
    
    Forgot that in cea2d71e641

diff --git a/src/insets/InsetExternal.h b/src/insets/InsetExternal.h
index 3cb295b..00546fa 100644
--- a/src/insets/InsetExternal.h
+++ b/src/insets/InsetExternal.h
@@ -96,6 +96,9 @@ class RenderBase;
 ///
 class InsetExternal : public Inset, public boost::signals::trackable
 {
+       // Disable assignment operator, since it is not used, and it is too
+       // complicated to implement it consistently with the copy constructor
+       InsetExternal & operator=(InsetExternal const &);
 public:
        InsetExternal(Buffer *);
        ///