Revert "dtc: install libfdt library and headers for host variant"

This reverts commit 5ca9b7cf. Installing
dtc headers in $(HOST_DIR) causes some build failures of
host-uboot-tools due to a conflicting libfdt.h header.

The possible solution to solve this problem, based on -isystem, has been
reverted has it was causing too many issues that we didn't want to solve
so close to cutting 2016.08-rc1.

Since the headers of libfdt for the host are currently not needed for
any package in the Buildroot tree, it is easier and safer to just revert
this patch.

Fixes:

  http://autobuild.buildroot.net/results/9fba77f4080bf6bc26e341f3f383cdba26c0e14f/



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