build: Add missing slash at end of include dir

[BlaiseLebreton]

Description

Add a slash at the end of CMAKE_INSTALL_INCLUDEDIR, otherwise when setting APPEND_PROJECT_NAME_TO_INCLUDEDIR to ON, the install path is : prefix.../includefastcdr/

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• Any new/modified methods have been properly documented using Doxygen.
• Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• Changes are API compatible.
• New feature has been added to the versions.md file (if applicable).
• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• Check CI results: changes do not issue any warning.
• Check CI results: CI pass and failing tests are unrelated with the changes.