Fix error C3016 on MSVC with OpenMP

[UnaNancyOwen]

This pull request was fixed error C3016 (issue #1499).

MSVC doesn't support an OpenMP versions earlier higher than 2.0.
The index variable in an OpenMP 2.0 for statement must be a signed integral type.
The error C3016 occurs on MSVC, because std::size_t is unsigned integral type.
Fix this issue by replacing index to the 64bit signed integral type.

[taketwo]

Why specifically long long, not int32_t or int64_t?

[UnaNancyOwen]

I think that int32_t and int64_t has been implemented from C++11.
Are you sure PCL want to use the C++11?

[taketwo]

On pre-c++11 compilers these types are available through Boost. We import them in pcl_macros.h and use here and there in the rest of PCL, so should be no problem.

[UnaNancyOwen]

OK, I changed for-loop index to int64_t.

[taketwo]

Thanks!

[UnaNancyOwen]

@taketwo Why closed this pull-request? Don't merge this commit?

[taketwo]

Ooooops, sorry. Thanks for the heads-up ;)

[UnaNancyOwen]

Thanks.