structures: Fix TitleDescription constructor

It only needs a const libvlc_title_description_t*.

vlcpp/structures.hpp

@@ -562,7 +562,7 @@ public:
         return ( m_flags & libvlc_title_interactive ) != 0;
     }
 
-    explicit TitleDescription( libvlc_title_description_t* c )
+    explicit TitleDescription( const libvlc_title_description_t* c )
         : m_duration( c->i_duration ), m_flags( c->i_flags )
     {
         if ( c->psz_name != nullptr )