[POWERPC] 4xx: Fix Walnut build

The addition of of_rtc for the Walnut board was only half complete.  Select
OF_RTC in the Kconfig and include the appropriate header to make it compile.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>

arch/powerpc/platforms/40x/Kconfig

@@ -72,6 +72,7 @@ config WALNUT
 	default y
 	select 405GP
 	select PCI
+	select OF_RTC
 	help
 	  This option enables support for the IBM PPC405GP evaluation board.
 

arch/powerpc/platforms/40x/walnut.c

@@ -18,6 +18,7 @@
 
 #include <linux/init.h>
 #include <linux/of_platform.h>
+#include <linux/rtc.h>
 
 #include <asm/machdep.h>
 #include <asm/prom.h>