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

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

This reverts commit 9fc96c7c19df ("selftests: error out if kernel header
files are not yet built").

It turns out that requiring the kernel headers to be built as a
prerequisite to building selftests, does not work in many cases. For
example, Peter Zijlstra writes:

"My biggest beef with the whole thing is that I simply do not want to use
'make headers', it doesn't work for me.

I have a ton of output directories and I don't care to build tools into
the output dirs, in fact some of them flat out refuse to work that way
(bpf comes to mind)." [1]

Therefore, stop erroring out on the selftests build. Additional patches
will be required in order to change over to not requiring the kernel
headers.

[1] https://lore.kernel.org/20231208221007.GO28727@noisy.programming.kicks-ass.net

Link: https://lkml.kernel.org/r/20231209020144.244759-1-jhubbard@nvidia.com
Fixes: 9fc96c7c19df ("selftests: error out if kernel header files are not yet built")
Signed-off-by: John Hubbard <jhubbard@nvidia.com>
Cc: Anders Roxell <anders.roxell@linaro.org>
Cc: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: David Hildenbrand <david@redhat.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>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Marcos Paulo de Souza <mpdesouza@suse.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>