Replace std::ignore with boost::ignore_unused()

According to https://isocpp.org/blog/2016/09/quick-q-stdignore-for-ignoring-unused-variable std::ignore is not supposed to be used for this

src/App/Metadata.cpp

@@ -25,6 +25,7 @@
 #ifndef _PreComp_
 # include <memory>
 # include <sstream>
+# include <boost/core/ignore_unused.hpp>
 #endif
 
 #include "Metadata.h"
@@ -61,7 +62,7 @@ namespace MetadataInternal {
         void warning(const SAXParseException& toCatch)
         {
             // Don't deal with warnings at all
-            std::ignore = toCatch;
+            boost::ignore_unused(toCatch);
         }
 
         void error(const SAXParseException& toCatch)