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

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

cpu/arm920t/s3c24x0/timer.c

diff --git a/cpu/arm920t/s3c24x0/timer.c b/cpu/arm920t/s3c24x0/timer.c
index 20cedd4..a27f0e2 100644
--- a/cpu/arm920t/s3c24x0/timer.c
+++ b/cpu/arm920t/s3c24x0/timer.c
@@ -205,6 +205,8 @@
 	struct s3c24x0_watchdog *watchdog;
 
 #ifdef CONFIG_TRAB
+	extern void disable_vfd(void);
+
 	disable_vfd();
 #endif