kbuild: Fix destination-y for installed headers

Commit 10b63956

 which plumbed in UAPI
broke the destination-y functionality of scripts/Makefile.headersinst.

The variable destination-y is used in a := assignment and so is expanded at
declaration time, and the include of the Kbuild fragments that set
destination-y to something is after this time, so it now always expands empty.

There are no in-tree users of destination-y, but it allows any
Kbuild-fragment to redirect where headers are installed.

Just move the assignment of the variable that uses it below the include
of the Kbuild fragment.

Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>