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>

configure

@@ -2344,6 +2344,7 @@ if test "$nettle" != "no"; then
         nettle="yes"
 
         cat > $TMPC << EOF
+#include <stddef.h>
 #include <nettle/pbkdf2.h>
 int main(void) {
      pbkdf2_hmac_sha256(8, NULL, 1000, 8, NULL, 8, NULL);