package/dtc: qemu system build need libfdt

The "install-bin" target just installs dtc's programs to host/bin
without libfdt, but building qemu system requires libfdt.

see: https://gitlab.com/c-sky/buildroot/-/jobs/38654503



So we need to install libfdt to satisfy the requirements of qemu.

Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>