blackfin: fixing warning by including proper headers

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
diff --git a/arch/blackfin/lib/board.c b/arch/blackfin/lib/board.c
index 17d1f46..392d72d 100644
--- a/arch/blackfin/lib/board.c
+++ b/arch/blackfin/lib/board.c
@@ -19,6 +19,7 @@
 #include <net.h>
 #include <status_led.h>
 #include <version.h>
+#include <watchdog.h>
 
 #include <asm/cplb.h>
 #include <asm/mach-common/bits/mpu.h>