With glibc 2.27 memfd_create() is behind a _GNU_SOURCE guard, so use
AC_USE_SYSTEM_EXTENSIONS to define it.
(From OE-Core rev: 88b3d730021107985ea749c92e52a323690f87dc)
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>