i2c-piix4: Fix build failure

Fix build failure in Intel PIIX4 I2C driver.
Signed-off-by: Daniel J Blueman <daniel@quora.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: stable@vger.kernel.org [v3.6]

drivers/i2c/busses/i2c-piix4.c

@@ -37,6 +37,7 @@
 #include <linux/stddef.h>
 #include <linux/ioport.h>
 #include <linux/i2c.h>
+#include <linux/slab.h>
 #include <linux/init.h>
 #include <linux/dmi.h>
 #include <linux/acpi.h>