Put _kernel.h in the correct path

Previously _kernel.h landed in pkgincludedir instead of in
pkgdatadir/include. The rationale behind this is, that headers required
by pocl should go to pkgdatadir/include, where as headers required by
3rd parties to build against pocl go into pkgcinludedir.

Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>
Fix the installation of headers required by the kernel compiler.