dtc: install libfdt library and headers for host variant

The dtc package currently does not install libfdt for the host install.
It can be useful to have libfdt on the host, such as for building QEMU
with the --enable-fdt configure switch.

Signed-off-by: David Raeman <draeman@bbn.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>