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/[email protected]/

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: John Hubbard <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Tested-by: Muhammad Usama Anjum <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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