Fix compile for gcc 12

GCC 12 has a bug that evaluated always false branches in templates,
leading to an annoying error.  This doesn't happen in GCC 13 or Clang.
To work around this in GCC 12, we provide a conditional expression that
does not depend on the template so GCC 12 is happy.

common/xnode.cpp

@@ -77,7 +77,7 @@ XATTR::XATTR( const wxString& aName, const VALUE_TYPE& aValue ) :
                     }
                     else
                     {
-                        static_assert( !std::is_same_v<T, T>, "Missing type handling in XNODE::FormatContents" );
+                        static_assert( sizeof( T ) == 0, "Missing type handling in XNODE::FormatContents" );
                     }
                     }, aValue );
 }