[libc] Fix the rtconfig issue in armlibc/unistd.h file.

components/libc/armlibc/sys/unistd.h

@@ -1,6 +1,8 @@
 #ifndef _SYS_UNISTD_H
 #define _SYS_UNISTD_H
 
+#include <rtthread.h>
+
 #ifdef RT_USING_DFS
 #include <dfs_posix.h>
 #else