bench: Avoid deprecated use of volatile +=

Author: MarcoFalke

src/bench/examples.cpp

@@ -13,7 +13,7 @@ static void Trig(benchmark::Bench& bench)
 {
     double d = 0.01;
     bench.run([&] {
-        sum += sin(d);
+        sum = sum + sin(d);
         d += 0.000001;
     });
 }