Makefile: add headers_install to kselftest targets

Add headers_install as a dependency to kselftest targets so that they
can be run directly from the top of the tree.  The kselftest Makefile
used to try to call headers_install "backwards" but failed due to the
relative path not being consistent.

Now we can either run this directly:

  $ make O=build kselftest-all

or this:

  $ make O=build headers_install
  $ make O=build -C tools/testing/selftest all

The same commands work as well when building directly in the source
tree (no O=) or any arbitrary path (relative or absolute).
Signed-off-by: NGuillaume Tucker <guillaume.tucker@collabora.com>
Tested-by: NAnders Roxell <anders.roxell@linaro.org>
Signed-off-by: NShuah Khan <skhan@linuxfoundation.org>

Makefile

@@ -1347,10 +1347,10 @@ tools/%: FORCE
 # Kernel selftest
 
 PHONY += kselftest
-kselftest:
+kselftest: headers_install
 	$(Q)$(MAKE) -C $(srctree)/tools/testing/selftests run_tests
 
-kselftest-%: FORCE
+kselftest-%: headers_install FORCE
 	$(Q)$(MAKE) -C $(srctree)/tools/testing/selftests $*
 
 PHONY += kselftest-merge