benchmark

Added Wconversion so the gcc/clang warnings will be more similar to W4 on MSVC.

This also points out some important issues like:
warning: conversion to 'size_t {aka unsigned int}' from 'long long int' may alter its value [-Wconversion]
   state.SetBytesProcessed(items_processed * sizeof(v));
This occurs on 32 bit systems where size_t is only 32 bits.