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>

scripts/Makefile.headersinst

@@ -7,15 +7,15 @@
 #
 # ==========================================================================
 
-# called may set destination dir (when installing to asm/)
-_dst := $(or $(destination-y),$(dst),$(obj))
-
 # generated header directory
 gen := $(if $(gen),$(gen),$(subst include/,include/generated/,$(obj)))
 
 kbuild-file := $(srctree)/$(obj)/Kbuild
 include $(kbuild-file)
 
+# called may set destination dir (when installing to asm/)
+_dst := $(or $(destination-y),$(dst),$(obj))
+
 old-kbuild-file := $(srctree)/$(subst uapi/,,$(obj))/Kbuild
 ifneq ($(wildcard $(old-kbuild-file)),)
 include $(old-kbuild-file)