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/sendcoinsentry.cpp

@@ -114,7 +114,7 @@ void SendCoinsEntry::clear()
 
 void SendCoinsEntry::deleteClicked()
 {
-    emit removeEntry(this);
+    Q_EMIT removeEntry(this);
 }
 
 bool SendCoinsEntry::validate()