LibC: Implement {f,}putwc()

Author: https://github.com/alimpfard
Commit: cb90856756
Pull-request: https://github.com/SerenityOS/serenity/pull/11324
Reviewed-by: https://github.com/BertalanD

Userland/Libraries/LibC/stdio.cpp

@@ -341,6 +341,11 @@ FILE::Buffer::~Buffer()
         free(m_data);
 }
 
+bool FILE::Buffer::may_use() const
+{
+    return m_ungotten || m_mode != _IONBF;
+}
+
 void FILE::Buffer::realize(int fd)
 {
     if (m_mode == -1)