Fix build with clang

In file included from /home/apol/devel/frameworks/kalgebra/src/dictionary.cpp:21:
.../include/Analitza5/analitza/expression.h:120:39: error: 'nodiscard' attribute cannot be applied to types
        Expression lambdaBody() const Q_REQUIRED_RESULT;
                                      ^
.../include/QtCore/qcompilerdetection.h:1177:31: note: expanded from macro 'Q_REQUIRED_RESULT'
 #  define Q_REQUIRED_RESULT [[nodiscard]]
                              ^