sensors: UASProximityDistance is missing an "unknown" enum value

[Martin Pitt]

include/ubuntu/application/sensors/event/proximity.h defines

    typedef enum {
        U_PROXIMITY_NEAR = 1, /**< The reading indicates that something is near the sensor/device. */
        U_PROXIMITY_FAR = 2 /**< The reading indicates that something is far away from the sensor/device. */
    } UbuntuApplicationSensorsProximityDistance;

This is missing a "U_PROXIMITY_UNKNOWN = 0" value, for an initial sensor value or other conditions where the value cannot be determined. I guess you even reserved 0 for something like that, but it should get an explicit enum value to avoid ugly type casts in API users.