Fix stdout_sink_base::log's behavior inconsistency

It will flush every time when it if not defined _WIN32, but not in
Windows family.
We viewed the commit #48d4ed9 for fixing issue #1675 .
It seems missing this flushing operation in mistake.

include/spdlog/sinks/stdout_sinks-inl.h

@@ -68,6 +68,7 @@ SPDLOG_INLINE void stdout_sink_base<ConsoleMutex>::log(const details::log_msg &m
     {
         throw_spdlog_ex("stdout_sink_base: WriteFile() failed. GetLastError(): " + std::to_string(::GetLastError()));
     }
+    FlushFileBuffers(handle_); // flush every line to terminal
 #else
     std::lock_guard<mutex_t> lock(mutex_);
     memory_buf_t formatted;