configure: Fix compiler warning in config.log (undefined NULL)

Avoid the warning when probing for xfs.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

diff --git a/configure b/configure
index e3713af..e9725d4 100755 (executable)
--- a/configure
+++ b/configure
@@ -1670,6 +1670,7 @@ fi
 # xfsctl() probe, used for raw-posix
 if test "$xfs" != "no" ; then
   cat > $TMPC << EOF
+#include <stddef.h>  /* NULL */
 #include <xfs/xfs.h>
 int main(void)
 {