Open RT_USING_POSIX_STDIO caused serial_v2 to compile failed (#5684)
Open RT_USING_POSIX_STDIO caused serial_v2 to compile failed (#5684)

* fix:Open RT_USING_POSIX_STDIO caused serial_v2 to compile failed
nongxiaoming/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,nongxiaoming/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread