bsp: cvitek: c906_littel: fixed build warnings for board.c

When building bsp/cvitek/c906_little, compiler warns:

```
board/board.c: In function 'rt_hw_board_init':
board/board.c:26:5: warning: implicit declaration of
function 'rt_hw_tick_init'; did you mean 'rt_hw_stack_init'?
[-Wimplicit-function-declaration]
   26 |     rt_hw_tick_init();
      |     ^~~~~~~~~~~~~~~
      |     rt_hw_stack_init
board/board.c:29:5: warning: implicit declaration of
function 'rt_hw_uart_init'; did you mean 'rt_hw_board_init'?
[-Wimplicit-function-declaration]
   29 |     rt_hw_uart_init();
      |     ^~~~~~~~~~~~~~~
      |     rt_hw_board_init
```

To remove these build warnings, include header files
which declare these functions.

Plus, remove the decalartion of `tick_isr()`, this
function does not exist.

Signed-off-by: Chen Wang <unicorn_wang@outlook.com>

Author: unicornx

bsp/cvitek/c906_little/board/board.c

@@ -11,6 +11,8 @@
 #include <rtthread.h>
 
 #include "board.h"
+#include "tick.h"
+#include "drv_uart.h"
 
 void rt_hw_board_init(void)
 {

bsp/cvitek/c906_little/board/tick.h

@@ -11,8 +11,6 @@
 #ifndef __TICK_H__
 #define __TICK_H__
 
-
-int tick_isr(void);
 int rt_hw_tick_init(void);
 
 #endif /* __TICK_H__ */