[ORC] Fix -Wunused-but-set-variable in JITLoaderPerf.cpp (NFC)

/data/llvm-project/llvm/lib/ExecutionEngine/Orc/TargetProcess/JITLoaderPerf.cpp:118:10: error: variable 'Written' set but not used [-Werror,-Wunused-but-set-variable]
  size_t Written = 0;
         ^
1 error generated.