selftests: Compile kselftest headers with -D_GNU_SOURCE

Add the -D_GNU_SOURCE flag to KHDR_INCLUDES so that it is defined in a
central location.

Commit 80921623 ("selftests/harness: remove use of LINE_MAX")
introduced asprintf into kselftest_harness.h, which is a GNU extension
and needs _GNU_SOURCE to either be defined prior to including headers or
with the -D_GNU_SOURCE flag passed to the compiler.

Fixed up commit log:
Shuah Khan <skhan@linuxfoundation.org>

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>
Reviewed-by: Mark Brown <broonie@kernel.org>
Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>