[cmake] Do NOT strip full paths:

It is essential to keep the notion of non-relocatable headers, passed to
ROOT_GENERATE_DICTIONARY with full paths. The dictionary will remember them
with their original location, making sure that headers can be found at
runtime without -I or $ROOT_INCLUDE_PATH.

ROOT's headers will never be passed as full path; users as well as ROOT
itself (in contrast to the non-relocatable roottest) MUST pass relative
paths, as if they were to #include the header, together with a matching -I.