selftests: error out if kernel header files are not yet built

selftests: error out if kernel header files are not yet built

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

Change the selftest build system's lib.mk to fail out with a helpful
message if that prerequisite "make headers" has not been done yet.

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

[[email protected]: abort the make process the first time headers aren't detected]
Link: https://lkml.kernel.org/r/[email protected]
[[email protected]: fix out-of-tree builds]
Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: John Hubbard <[email protected]>
Signed-off-by: Anders Roxell <[email protected]>
Reviewed-by: Muhammad Usama Anjum <[email protected]>
Tested-by: Muhammad Usama Anjum <[email protected]>
Cc: David Hildenbrand <[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]>