Re-add deprecated symbol for ABI compatibility with Qt 5.1.

When b0b786a2f0 deprecated
QFontDatabase::supportsThreadedFontRendering() it made the
function inline also. That way compilers like gcc did not
export the symbol anymore.

Change-Id: If9c343eaa2ff90540decbc19b0af33d439c0bbaa
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/gui/text/qfontdatabase.h

@@ -151,7 +151,7 @@ public:
     static bool removeAllApplicationFonts();
 
 #if QT_DEPRECATED_SINCE(5, 2)
-    QT_DEPRECATED static inline bool supportsThreadedFontRendering() { return true; }
+    QT_DEPRECATED static bool supportsThreadedFontRendering();
 #endif
 
     static QFont systemFont(SystemFont type);

src/gui/text/qfontdatabase_qpa.cpp

@@ -267,6 +267,12 @@ bool QFontDatabase::removeAllApplicationFonts()
     return true;
 }
 
+// QT_DEPRECATED_SINCE(5, 2)
+bool QFontDatabase::supportsThreadedFontRendering()
+{
+    return true;
+}
+
 /*!
     \internal
 */