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/bitcoinamountfield.cpp

@@ -61,7 +61,7 @@ public:
     void setValue(const CAmount& value)
     {
         lineEdit()->setText(BitcoinUnits::format(currentUnit, value, false, BitcoinUnits::separatorAlways));
-        emit valueChanged();
+        Q_EMIT valueChanged();
     }
 
     void stepBy(int steps)
@@ -184,7 +184,7 @@ protected:
         return rv;
     }
 
-signals:
+Q_SIGNALS:
     void valueChanged();
 };