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

/llvm-project/llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp:415:18:
error: unused variable 'CurrentCPU' [-Werror,-Wunused-variable]
    unsigned int CurrentCPU;
                 ^
1 error generated.

Author: DamonFool

llvm/tools/llvm-exegesis/lib/BenchmarkRunner.cpp

@@ -412,7 +412,7 @@ class SubProcessFunctionExecutorImpl
 
     // Check (if assertions are enabled) that we are actually running on the
     // CPU that was specified by the user.
-    unsigned int CurrentCPU;
+    [[maybe_unused]] unsigned int CurrentCPU;
     assert(getcpu(&CurrentCPU, nullptr) == 0 &&
            "Expected getcpu call to succeed.");
     assert(static_cast<int>(CurrentCPU) == CPUToUse &&