Specify MONGO_LOG_DEFAULT_COMPONENT for WT engine

You'll need this as soon as you merge upstream commit https://github.com/mongodb/mongo/commit/5fc306543cd3ba2637e5cb0662cc375f36868b28

Any cpp file which imports "mongo/util/log.h" now must specify MONGO_LOG_DEFAULT_COMPONENT or the preprocessor will #error. See https://jira.mongodb.org/browse/SERVER-15592 for details.