PM/Qos: Expose dev_pm_qos_flags symbol

The dev_pm_qos_flags() will be used in the usb core which could be
compiled as a module. This patch is to export it.

Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>