[llvm-exegesis] Fix -Wc++98-compat-extra-semi in BenchmarkRunner.cpp (NFC)

/data/llvm-project/llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp:66:2: error: extra ';' outside of a function is incompatible with C++98 [-Werror,-W
c++98-compat-extra-semi]
};
 ^
1 error generated.