rm48x50: fix the prototype of finsh_system_init

bsp/rm48x50/application/startup.c

@@ -19,7 +19,7 @@
 
 #ifdef RT_USING_FINSH
 #include <finsh.h>
-extern void finsh_system_init(void);
+extern int finsh_system_init(void);
 #endif
 
 #include <board.h>