of: fix missing include from of_pci.c

of_pci.c references symbols from linux/of.h.
Signed-off-by: NGrant Likely <grant.likely@secretlab.ca>

drivers/of/of_pci.c

 #include <linux/kernel.h>
+#include <linux/of.h>
 #include <linux/of_pci.h>
 #include <asm/prom.h>