writeback: remove a duplicate prototype for tag_pages_for_writeback

writeback: remove a duplicate prototype for tag_pages_for_writeback

[hch@lst.de: split from a larger patch]
Link: https://lkml.kernel.org/r/20240215063649.2164017-3-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Acked-by: Dave Chinner <dchinner@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>