trab: fix warning: implicit declaration of function 'disable_vfd'

Signed-off-by: Wolfgang Denk <wd@denx.de>

cpu/arm920t/s3c24x0/timer.c

@@ -205,6 +205,8 @@ void reset_cpu(ulong ignored)
 	struct s3c24x0_watchdog *watchdog;
 
 #ifdef CONFIG_TRAB
+	extern void disable_vfd(void);
+
 	disable_vfd();
 #endif