Makefile: replace headers_install with headers for kselftest

Replace headers_install with headers as kselftest uses the header
files from within the kernel tree rather than from a system-wide
installation.

We can still run this directly:

  $ make O=build kselftest-all

and when building from the selftests directory:

  $ make O=build headers
  $ make O=build -C tools/testing/selftests all
Signed-off-by: Guillaume Tucker <guillaume.tucker@collabora.com>
Reported-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Makefile

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