core: Make logger thread-safe

Change-Id: Ic45dc0912da9c2e3f074d3f74921bfe33741bf63
Refs: #2489
diff --git a/core/logger-factory.hpp b/core/logger-factory.hpp
index 9f4a9f3..606ac1c 100644
--- a/core/logger-factory.hpp
+++ b/core/logger-factory.hpp
@@ -101,6 +101,7 @@
   typedef std::pair<std::string, Logger> NameAndLogger;
 
   LoggerMap m_loggers;
+  mutable std::mutex m_loggersGuard;
 
   LogLevel m_defaultLevel;
 };