src/main/resources is the “standard placement” for this.
Update: The above answers the question, but its not the best solution. Check out the other answers and the comments on this … you would probably not shipping your own logging properties with the jar but instead leave it to the client (for example app-server, stage environment, etc) to configure the desired logging. Thus, putting it in src/test/resources is my preferred solution.
Note: Speaking of leaving the concrete log config to the client/user, you should consider replacing log4j with slf4j in your app.