Use initializer list to safely initialize a static in ELseverityLevel

The function loadMap is callable from multiple threads and therefore the initialization of the map must be done in a safe manner. Using an initializer list will guarantee that the compiler will make the initialization of the map thread-safe.
This was found by the static analyzer.
Merge pull request #4426 from Dr15Jones/threadSafeInitOfStaticELseverityLevel

Use initializer list to safely initialize a static in ELseverityLevel