Fix implicit declaration build warnings

- warning: implicit declaration of function ‘serial_initialize’

Signed-off-by: Marcel Ziswiler <marcel@ziswiler.com>

lib_arm/board.c

@@ -44,6 +44,7 @@
 #include <devices.h>
 #include <version.h>
 #include <net.h>
+#include <serial.h>
 
 #ifdef CONFIG_DRIVER_SMC91111
 #include "../drivers/net/smc91111.h"

lib_m68k/board.c

@@ -45,6 +45,7 @@
 #include <status_led.h>
 #endif
 #include <net.h>
+#include <serial.h>
 #if defined(CONFIG_CMD_BEDBUG)
 #include <cmd_bedbug.h>
 #endif