selftests/sgx: Include KHDR_INCLUDES in Makefile

Add KHDR_INCLUDES to the CFLAGS to pull in the kselftest harness
dependencies (-D_GNU_SOURCE).

Also, remove redefinitions of _GNU_SOURCE in the source code.

Fixes: 80921623 ("selftests/harness: remove use of LINE_MAX")
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202404301040.3bea5782-oliver.sang@intel.com


Signed-off-by: Edward Liaw <edliaw@google.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>