[llvm-exegesis] Fix -Wunused-variable in BenchmarkRunner.cpp (NFC)

/data/llvm-project/llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp:275:9: error: unused variable 'ParentPIDFD' [-Werror,-Wunused-variable]
    int ParentPIDFD = syscall(SYS_pidfd_open, ParentPID, 0);
        ^
1 error generated.