sensor_monitor: Watch for interfacesRemoved

Now that the hwmontempsensor app can remove sensors from D-Bus when the
entity-manager config goes away, the sensor monitor needs to deal with
it.  It does so by watching for a threshold interface to be removed from
a sensor and removing that entry from its map of alarms to watch.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I6b48b20eb4d158e5e44566e46469421550d21686
diff --git a/sensor-monitor/threshold_alarm_logger.hpp b/sensor-monitor/threshold_alarm_logger.hpp
index 3380e49..a43191a 100644
--- a/sensor-monitor/threshold_alarm_logger.hpp
+++ b/sensor-monitor/threshold_alarm_logger.hpp
@@ -187,6 +187,11 @@
     sdbusplus::bus::match::match perfLossMatch;
 
     /**
+     * @brief The InterfacesRemoved match object
+     */
+    sdbusplus::bus::match::match ifacesRemovedMatch;
+
+    /**
      * @brief The current alarm values
      */
     std::map<InterfaceKey, std::map<PropertyName, bool>> alarms;