selftests: add and export a kernel uapi headers path

selftests: add and export a kernel uapi headers path

Kernel uapi headers can be present at different paths depending upon how
the build was invoked.  It becomes impossible for the tests to include the
correct headers directory.  Set and export KHDR_INCLUDES variable to make
it possible for sub make files to include the header files.

Link: https://lkml.kernel.org/r/20220119101531.2850400-3-usama.anjum@collabora.com
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andr Almeida <andrealmeid@collabora.com>
Cc: Darren Hart <dvhart@infradead.org>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Mat Martineau <mathew.j.martineau@linux.intel.com>
Cc: Matthieu Baerts <matthieu.baerts@tessares.net>
Cc: Mickal Salan <mic@digikod.net>
Cc: Minghao Chi <chi.minghao@zte.com.cn>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>