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.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index b799a62..1e933d2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -37,6 +37,7 @@
 else()

   add_cxx_compiler_flag(-Wall)

 endif()
+add_cxx_compiler_flag(-Wconversion)
 add_cxx_compiler_flag(-Wextra)
 add_cxx_compiler_flag(-Wshadow)
 add_cxx_compiler_flag(-Werror RELEASE)