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/guiutil.h

@@ -169,7 +169,7 @@ namespace GUIUtil
             void setViewHeaderResizeMode(int logicalIndex, QHeaderView::ResizeMode resizeMode);
             void resizeColumn(int nColumnIndex, int width);
 
-        private slots:
+        private Q_SLOTS:
             void on_sectionResized(int logicalIndex, int oldSize, int newSize);
             void on_geometriesChanged();
     };