Don't join absolute paths

The intention here is to point to the /usr/include in the sysroot,
however when joining an absolute paths the result will just be the last
absolute paths

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Guillaume Desmottes <guillaume.desmottes@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D5256