Documentation: kselftest: "make headers" is a prerequisite

As per a discussion with Muhammad Usama Anjum [1], the following is how
one is supposed to build selftests:

    make headers && make -C tools/testing/selftests/mm

However, that's not yet documented anywhere. So add it to
Documentation/dev-tools/kselftest.rst .

[1] https://lore.kernel.org/all/bf910fa5-0c96-3707-cce4-5bcc656b6274@collabora.com/

Link: https://lkml.kernel.org/r/20230606071637.267103-11-jhubbard@nvidia.comSigned-off-by: John Hubbard <jhubbard@nvidia.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Tested-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Documentation/dev-tools/kselftest.rst

@@ -36,6 +36,7 @@ Running the selftests (hotplug tests are run in limited mode)
 
 To build the tests::
 
+  $ make headers
   $ make -C tools/testing/selftests
 
 To run the tests::