logging: Fix silent logs dropping when LOG_MODE_OVERFLOW disabled

There was no notification about dropped logs When logger operated
in the mode where message is dropped when logger has no space to
store the message. Notification was printed only if logger operated
in the mode which overwrites oldest log.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>

include/logging/log_core.h

@@ -540,6 +540,10 @@ bool log_is_strdup(void *buf);
  */
 void log_free(void *buf);
 
+/** @brief Indicate to the log core that one log message has been dropped.
+ */
+void log_dropped(void);
+
 #ifdef __cplusplus
 }
 #endif