genksyms: fix syntax error for attribute after 'union'

A longstanding issue with genksyms is that it has hidden syntax errors.

When a syntax error occurs, yyerror() is called. However,
error_with_pos() is a no-op unless the -w option is provided.

You can observe syntax errors by manually passing the -w option.

For example, with CONFIG_MODVERSIONS=y on v6.13-rc1:

    $ make -s KCFLAGS=-D__GENKSYMS__ fs/lockd/svc.i
    $ cat fs/lockd/svc.i | scripts/genksyms/genksyms -w
        [ snip ]
    ./include/net/addrconf.h:35: syntax error

The syntax error occurs in the following code in include/net/addrconf.h:

    union __packed {
            [ snip ]
    };

The issue arises from __packed, which is defined as
__attribute__((__packed__)), immediately after the 'union' keyword.

This commit allows the 'union' keyword to be followed by attributes.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nicolas Schier <n.schier@avm.de>