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

When b0b786a2f05e9451a65519ab8904f55c35f51b7d 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>