of: Fix missing include

Fix a build failure on ARM
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>

drivers/of/platform.c

@@ -15,6 +15,7 @@
 #include <linux/module.h>
 #include <linux/device.h>
 #include <linux/dma-mapping.h>
+#include <linux/slab.h>
 #include <linux/of_device.h>
 #include <linux/of_platform.h>