selftests: ptrace: Use installed kernel headers search path

Use $(KHDR_INCLUDES) as lookup path for installed kernel headers rather
than using kernel headers in include/uapi from the source kernel tree
kernel headers.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: linux-kselftest@vger.kernel.org
Cc: Ingo Molnar <mingo@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>