dtc: install libfdt headers to staging

The newly introduced install-lib target that we use since commit 8b500c1b
(dtc: bump version to 1.4.1) does not install headers. Add install-includes to
DTC_INSTALL_STAGING_CMDS to make dependent packages like kexec-lite build.

Fixes:
http://autobuild.buildroot.net/results/515/515d32e45957e911fda6345f909fa9200df311ff/
http://autobuild.buildroot.net/results/b09/b09f15c7b59e972d2180d071804810315d2ee047/
http://autobuild.buildroot.net/results/023/02376cd3dccc98d10a58e5398b9f8db75239f263/


...

Cc: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>