Introduce Q_FALLTHROUGH()

Silence g++ 7.X warnings.

Change-Id: I9d06d04b496c9ec060e13e1be6f43d8fbadb1f3b
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cpptools/cppqtstyleindenter.cpp

@@ -29,6 +29,8 @@
 #include "cpptoolssettings.h"
 #include "cppcodestylepreferences.h"
 
+#include <utils/qtcfallthrough.h>
+
 #include <QChar>
 #include <QTextDocument>
 #include <QTextBlock>
@@ -79,7 +81,7 @@ static bool isElectricInLine(const QChar ch, const QString &text)
             return true;
         }
 
-        // fall-through
+        Q_FALLTHROUGH();
         // lines that start with : might have a constructor initializer list
     case '<':
     case '>': {