Unwarn compile of QtBluetooth

warning: 'QLowEnergyDescriptorPrivate' defined as a struct here but previously declared as a class [-Wmismatched-tags]

Change-Id: I2b90a645ed5aa2d86184d94acb2658460f26f660
Reviewed-by: Timur Pocheptsov <Timur.Pocheptsov@digia.com>
Reviewed-by: Alex Blasche <alexander.blasche@digia.com>

src/bluetooth/qlowenergydescriptor.h

@@ -42,7 +42,7 @@
 
 QT_BEGIN_NAMESPACE
 
-class QLowEnergyDescriptorPrivate;
+struct QLowEnergyDescriptorPrivate;
 class QLowEnergyServicePrivate;
 
 class Q_BLUETOOTH_EXPORT QLowEnergyDescriptor