Visual Studio 15 warning in latest tip

[shinchiro]

cmake -G "Visual Studio 15 Win64" ../../source

(ClCompile target) ->
  D:\x265\source\encoder\encoder.cpp(2733): warning C4334: '<<': result of 32-bit shift implicitly converted to 64 bits (was 64-bit shift intended?) [D:\x265\build\vc14-x86_64\encoder\encoder.vcxproj]