Fix build error (NULL undefined) caused by missing include file

git-svn-id: http://lm-sensors.org/svn/i2c-tools/trunk@6124 7894878c-1315-0410-8ee3-d5d059ff63e0

CHANGES

@@ -32,6 +32,7 @@ SVN HEAD
   library: New libi2c library
            Properly propagate real error codes on read errors
            Use I2C_SMBUS_BLOCK_MAX instead of hard-coding 32
+  lib/smbus.c: Add missing include which was causing a build error
 
 3.1.0 (2011-12-04)
   decode-dimms: Decode module configuration type of DDR SDRAM

lib/smbus.c

@@ -22,6 +22,7 @@
 */
 
 #include <errno.h>
+#include <stddef.h>
 #include <i2c/smbus.h>
 #include <sys/ioctl.h>
 #include <linux/types.h>