kselftests: hid: fix missing headers_install step

The Makefile was assuming that headers_install was already done in
the top source directory, and was searching for installed uapi headers
there.
Unfortunately this is not the case and we need to manually call that step.

To do so, reorder the declaration of the variables, and reuses top_srcdir
provided by lib.mk

Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/all/202212060216.a6X8Py5H-lkp@intel.com/#t


Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Jiri Kosina <jkosina@suse.cz>
Link: https://lore.kernel.org/r/20221206145936.922196-6-benjamin.tissoires@redhat.com