[lib] Fix the compiling warning in libc.

components/libc/compilers/armlibc/libc.h

@@ -32,6 +32,6 @@ int libc_system_init(void);
 
 int libc_stdio_set_console(const char* device_name, int mode);
 int libc_stdio_read (void *buffer, size_t size);
-int libc_stdio_write(void *buffer, size_t size);
+int libc_stdio_write(const void *buffer, size_t size);
 
 #endif

components/libc/compilers/armlibc/stdio.c

@@ -63,7 +63,7 @@ int libc_stdio_read(void *buffer, size_t size)
     return read(std_fd, buffer, size);
 }
 
-int libc_stdio_write(void *buffer, size_t size)
+int libc_stdio_write(const void *buffer, size_t size)
 {
     return write(std_fd, buffer, size);
 }

components/libc/compilers/armlibc/stubs.c

@@ -31,6 +31,7 @@
 #include <rt_sys.h>
 
 #include "rtthread.h"
+#include "libc.h"
 
 #ifdef RT_USING_DFS
 #include "dfs_posix.h"