[c++] Silence unused warning in car benchmark.

Author: mjpt777

sbe-benchmarks/src/main/cpp/SbeCarCodecBench.h

@@ -133,6 +133,11 @@ class SbeCarCodecBench : public CodecBench<SbeCarCodecBench>
  tmpChar = car.manufacturer();
  tmpChar = car.model();
 
+ (void)tmpInt;
+ (void)tmpChar;
+ (void)tmpDouble;
+ (void)tmpBool;
+
  return car.encodedLength();
  };