Revert "selftests/sgx: Include KHDR_INCLUDES in Makefile"

This reverts commit 2c3b8f8f37.

The framework change to add D_GNU_SOURCE to KHDR_INCLUDES
to Makefile, lib.mk, and kselftest_harness.h is reverted
as it is causing build failures and warnings.

Revert this change as this change depends on the framework
change.

Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

tools/testing/selftests/sgx/sigstruct.c

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0
 /*  Copyright(c) 2016-20 Intel Corporation. */
 
+#define _GNU_SOURCE
 #include <assert.h>
 #include <getopt.h>
 #include <stdbool.h>