Fix eventType not used

If SEL_LOGGER_SEND_TO_LOGGING_SERVICE was not used, then during
compilation an error `error: variable 'eventType' set but not used`.
This patch fixes that bug.

Test: compilation with SEL_LOGGER_SEND_TO_LOGGING_SERVICE successfully
Compiling without SEL_LOGGER_SEND_TO_LOGGING_SERVICE is also successful

Change-Id: I214f1500e3a73582efe5a74d7aab0ca16e0b5c48
Signed-off-by: Glukhov Mikhail <mikl@greenfil.ru>
diff --git a/include/threshold_event_monitor.hpp b/include/threshold_event_monitor.hpp
index 423f8ff..6943d28 100644
--- a/include/threshold_event_monitor.hpp
+++ b/include/threshold_event_monitor.hpp
@@ -211,7 +211,7 @@
             eventWarn,
             eventErr
         };
-        EventType eventType = eventNone;
+        [[maybe_unused]] EventType eventType = eventNone;
         if (event == "CriticalLow")
         {
             threshold = "critical low";