Building qtwebengine failed with Yocto2.1/GCC5.3 with an IMX.6 ARM
target. Adding the include fixed the issue.

compiling .moc/moc_qquickwebengineview_p.cpp
.moc/moc_qquickwebengineview_p.cpp: In static member function 'static void QQuickWebEngineView::qt_static_metacall(QObject*, QMetaObject::Call, int, void**)':
.moc/moc_qquickwebengineview_p.cpp:866:72: error: invalid use of incomplete type 'class QColor'
         case 15: *reinterpret_cast< QColor*>(_v) = _t->backgroundColor(); break;
                                                                        ^
In file included from /data/projects/fsl-community-bsp/build/tmp/sysroots/wandboard/usr/include/qt5/QtCore/qobject.h:48:0,
                 from /data/projects/fsl-community-bsp/build/tmp/sysroots/wandboard/usr/include/qt5/QtCore/QObject:1,
                 from .moc/../api/qquickwebenginescript_p.h:52,
                 from .moc/../api/qquickwebengineview_p.h:52,
                 from .moc/moc_qquickwebengineview_p.cpp:9:
/data/projects/fsl-community-bsp/build/tmp/sysroots/wandboard/usr/include/qt5/QtCore/qmetatype.h:1886:1: note: forward declaration of 'class QColor'

Change-Id: If58315d4c0b5087705687ae582ffe86702063937
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>