Fix failure to build QtQuick with LTO

The presence of the inline QTestFontEngine's constructor causes a linker
failure because QFontEngineBox isn't exported. I'd say this is a
compiler bug (GCC 4.9), but it's an easy workaround and a difficult
testcase.

typeinfo for QTestFontEngine: error: undefined reference to 'typeinfo for QFontEngineBox'
vtable for QTestFontEngine: error: undefined reference to 'QFontEngineBox::glyphIndex(unsigned int) const'

Change-Id: I84829d111616977d6f3fcbbb48509d1c7d4f5fa6
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

src/gui/text/qfontengine.cpp

@@ -2105,4 +2105,8 @@ QFontEngine* QFontEngineMultiBasicImpl::createMultiFontEngine(QFontEngine *fe, i
     return engine;
 }
 
+QTestFontEngine::QTestFontEngine(int size)
+    : QFontEngineBox(TestFontEngine, size)
+{}
+
 QT_END_NAMESPACE

src/gui/text/qfontengine_p.h

@@ -463,7 +463,7 @@ private:
 class QTestFontEngine : public QFontEngineBox
 {
 public:
-    inline QTestFontEngine(int size) : QFontEngineBox(TestFontEngine, size) {}
+    QTestFontEngine(int size);
 };
 
 QT_END_NAMESPACE