call flush on the wrapped writebuffered FileFd

The flush call is a no-op in most FileFd implementations so this isn't
as critical as it might sound as the only non-trivial implementation is
in the buffered writer, which tends not be used to buffer another
buffer…

apt-pkg/contrib/fileutl.cc

@@ -1277,9 +1277,8 @@ public:
 
 	 writebuffer.bufferstart += written;
       }
-
       writebuffer.reset();
-      return true;
+      return wrapped->InternalFlush();
    }
    virtual ssize_t InternalWrite(void const * const From, unsigned long long const Size) APT_OVERRIDE
    {