[LyX/master] Make destructor virtual

[Georg Baum]

commit acb09a3efdedc997ddbe4c174eec48cedf9f0ad7
Author: Georg Baum <b...@lyx.org>
Date:   Fri Sep 11 23:04:59 2015 +0200

    Make destructor virtual
    
    Base classes with virtual methods should have a virtual destructor.
    See Scott Meyers, Effective C++ item 7: Declare destructors virtual in
    polymorphic base classes.

diff --git a/src/output_xhtml.h b/src/output_xhtml.h
index 76f7bd9..8ee5149 100644
--- a/src/output_xhtml.h
+++ b/src/output_xhtml.h
@@ -75,6 +75,8 @@ struct EndTag
 {
        ///
        explicit EndTag(std::string tag) : tag_(tag) {}
+       ///
+       virtual ~EndTag() {}
        /// </tag_>
        virtual docstring writeEndTag() const;
        ///