CMake: fix include directories

[nim65s]

Hi,

I currently have the following CMake error:

CMake Error in common/CMakeLists.txt:
  Target "libfranka-common" INTERFACE_INCLUDE_DIRECTORIES property contains
  path:
    "/build/source/common/include"
  which is prefixed in the source directory.

It seems that this was already fixed by @AndreasGFranka and committed by @tingelst almost one year ago, but this was not propagated to this main repo, so I guess this should be done.

I can't open a PR on this repo for that ("Pull request creation failed. Validation failed: must be a collaborator", which also appears to be a big issue by itself), so please pull tingelst@cacc912 or something similar.

Thanks !