Proper define of NDEBUG to avoid adding assertions in string benchmarks

bench/bench-string-2.cpp

@@ -22,6 +22,8 @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
+#define NDEBUG
+
 #include <string>
 #include <sstream>
 #include <string>