idl files need to be packaged relative to INSTDIR, not INSTROOT

...so that on OS X they end up in LibreOffice*_SDK.app/idl/, not
LibreOffice.app/Contents/LibreOffice*_SDK.app/idl/.

Change-Id: Id001aed334a0e1d8f835ce797a17f1f67d1a3abb
(cherry picked from commit f4ccd054a94de1290e01bd21d5d45f1720f7cb74)
Reviewed-on: https://gerrit.libreoffice.org/7569
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>