Introduce Q_FALLTHROUGH()

Silence g++ 7.X warnings.

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

src/plugins/fakevim/fakevimhandler.cpp

@@ -60,6 +60,7 @@
 
 #include <utils/hostosinfo.h>
 #include <utils/qtcassert.h>
+#include <utils/qtcfallthrough.h>
 
 #include <QDebug>
 #include <QFile>
@@ -5662,7 +5663,7 @@ bool FakeVimHandler::Private::handleExMapCommand(const ExCommand &cmd0) // :map
             foreach (char c, modes)
                 MappingsIterator(&g.mappings, c, key).remove();
             break;
-        case Map: // fall through
+        case Map: Q_FALLTHROUGH();
         case Noremap: {
             Inputs inputs(rhs, type == Noremap, silent);
             foreach (char c, modes)