Fix OpenMP compile issue under MSVC

[SunBlack]

Fix error C3016: 'i': index variable in OpenMP 'for' statement must have signed integral type

Issue occurs, as MSVC doesn't support size_t as OpenMP loop variable (see here).

[mvieth]

Needs a cast around size().
Do you know why this error didn't appear earlier? Do we not compile with openmp on the CI checks?
As a note: some other classes use std::ptrdiff_t for this purpose (but I am also fine with long long).

[SunBlack]

Thx for the hint with std::ptrdiff_t 👍

The compile issues is not happening on the CI, as there are no tests for TrajkovicKeypoint2D. The app pcl_ni_trajkovic (which requires OpenNI(1), which cannot be installed via vcpkg) is the only file which creates an instance of this class (as it is an template, there is no reason for the compiler to precompile it).

[mvieth]

Ah, I see. Thanks for the explanation.
However, vcpkg offers OpenNI2, which we currently don't install on the CI. Maybe we should do that? (independent from this error in TrajkovicKeypoint2D) @larshg