Fix EIGEN_ALIGN16 positionining for point type XYZHSV

[dooxe]

For point type XYZHSV structure, the EIGEN_ALIGN16 was positioned at the wrong place, leading to a compilation error.

[SergioRAgostinho]

I'm surprised this triggers a compilation error in your dev environment but not in the 3 separate platforms we have on the CI server. 😕

Note to maintainers: Don't forget to squash.

[dooxe]

Yes, I understand you disapointment. The placement of alignas modifier seems to be before the struct according to this doc:
https://en.cppreference.com/w/cpp/language/alignas
(I also found other examples of this syntax on MSDN doc and other pages).

From what I saw in PCL source code, some structures were corrected, some not.
Also, there is the same error in:

• convolution.h
• implicit_shape_model.h

They are not in this pull request because I saw them after reporting this ... :/

[SergioRAgostinho]

You can add them here as well. Push the extra commits to your branch.