kernel-doc: Add support for __deprecated

The current linux-next tree has a new error:

./Documentation/gpu/drm-mm:445: ./drivers/gpu/drm/drm_prime.c:994: WARNING: Error in declarator or parameters
Invalid C declaration: Expecting "(" in parameters. [error at 17]
  int __deprecated drm_prime_sg_to_page_array (struct sg_table *sgt, struct page **pages, int max_entries)
  -----------------^

While we might consider that documenting a deprecated interface is not
necessarily best practice, removing the error is easy.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Link: https://lore.kernel.org/r/20210427114828.GY235567@casper.infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/kernel-doc

@@ -1777,6 +1777,7 @@ sub dump_function($$) {
     $prototype =~ s/^noinline +//;
     $prototype =~ s/__init +//;
     $prototype =~ s/__init_or_module +//;
+    $prototype =~ s/__deprecated +//;
     $prototype =~ s/__flatten +//;
     $prototype =~ s/__meminit +//;
     $prototype =~ s/__must_check +//;