Fix implicit declaration build warnings

- warning: implicit declaration of function ‘serial_initialize’

Signed-off-by: Marcel Ziswiler <marcel@ziswiler.com>
diff --git a/lib_m68k/board.c b/lib_m68k/board.c
index 9159206..6654f97 100644
--- a/lib_m68k/board.c
+++ b/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