Fix build with Qt 5.6 / qtcreator.qbs

Broke with:

  Introduce Q_FALLTHROUGH()
  commit f482270432

Change-Id: I4f99f9a4d8a56ab1358be53f83aa0c449cff69a8
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/libs/qmljs/parser/qmljslexer.cpp

@@ -32,6 +32,8 @@
 #include <QtCore/qvarlengtharray.h>
 #include <QtCore/qdebug.h>
 
+#include <utils/qtcfallthrough.h>
+
 QT_BEGIN_NAMESPACE
 Q_CORE_EXPORT double qstrtod(const char *s00, char const **se, bool *ok);
 QT_END_NAMESPACE