CMake update to avoid directxmath subdir for include

[walbourn]

When using MSBuild+VCPKG, it required code changes in the client code to reference the DirectXMath headers properly which was not an issue for CMake+VCPKG.

This change removes the directxmath part of include/directxmath for the installed directory. Since these headers don't conflict with other ports, this works fine.

Note that directxtex, directxmesh, and uvatlas already follow this pattern (i.e. just install to include not a subdir). directxtk, directxk12 have the same header names, so they really have to be in subdirs.