qt: define QT_NO_KEYWORDS

QT_NO_KEYWORDS prevents Qt from defining the `foreach`, `signals`,
`slots` and `emit` macros.

Avoid overlap between Qt macros and boost - for example #undef hackiness
in #6421.

src/qt/optionsmodel.h

@@ -81,7 +81,7 @@ private:
     /// Add option to list of GUI options overridden through command line/config file
     void addOverriddenOption(const std::string &option);
 
-signals:
+Q_SIGNALS:
     void displayUnitChanged(int unit);
     void coinControlFeaturesChanged(bool);
 };