added macro guard to enable overriding ENABLE_LOG

The LOGLN macro is defined in header file only if ENABLE_LOG is 1. But ENABLE_LOG was explicitly defined to be 0 in the header without a macro guard so it was not possible to use the LOGLN macro. So i added the guard. This is used in the stitching_detailed.cpp sample code.
I am new to open source projects. This is my first contribution ever. Please let me know if I did something wrong. Thanks!