Apply InfoLogLevel to the logs in utilities/merge_operators/uint64add.cc

Summary:
Apply InfoLogLevel to the logs and add missing copy-right information
to  utilities/merge_operators/uint64add.cc.

Test Plan: make

Reviewers: ljin, sdong, igor

Reviewed By: igor

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D27897