Minor fix to allow build on non-Windows platforms

Build fails on Linux with the following error:
````
main.cpp
../../src/main.cpp: In function ‘void BenchCodeSize(const TestBase&, const TestJsonList&, FILE*)’:
../../src/main.cpp:676:9: error: ‘ret’ was not declared in this scope
     if (ret != 0) {
````
See attached fix.