Fix configure test for PBKDF2 in nettle

Fix configure test for PBKDF2 in nettle

On my Debian jessie system, including nettle/pbkdf2.h does not cause
NULL to be defined, which causes the test to fail to compile.  Include
stddef.h to bring in a definition of NULL.

Cc: qemu-trivial@nongnu.org
Cc: qemu-stable@nongnu.org
Signed-off-by: Steven Luo <steven+qemu@steven676.net>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
(cherry picked from commit 9e87a691bd46846e2232f8c30605c491c85ac987)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>