sensor-monitor: Fix missing function prototype

Add the missing ThresholdAlarmLogger::interfacesRemoved function
prototype.  This passed CI because the --enable-sensor-monitor flag
must not be turned on.

Signed-off-by: Matt Spinler <spinler@us.ibm.com>
Change-Id: I5ad5617bd56ae6649d6adde31faa02f9326d0483
diff --git a/sensor-monitor/threshold_alarm_logger.hpp b/sensor-monitor/threshold_alarm_logger.hpp
index a43191a..0067274 100644
--- a/sensor-monitor/threshold_alarm_logger.hpp
+++ b/sensor-monitor/threshold_alarm_logger.hpp
@@ -81,6 +81,16 @@
     void propertiesChanged(sdbusplus::message::message& msg);
 
     /**
+     * @brief The interfacesRemoved removed handler for the threshold
+     *        interfaces.
+     *
+     * Removes that threshold from the alarms map
+     *
+     * @param[in] msg - The signal message payload.
+     */
+    void interfacesRemoved(sdbusplus::message::message& msg);
+
+    /**
      * @brief Checks for active alarms on the path and threshold interface
      *        passed in and creates event logs if necessary.
      *