genksyms: fix syntax error for attribute before nested_declarator

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__ drivers/acpi/prmt.i
    $ cat drivers/acpi/prmt.i | scripts/genksyms/genksyms -w
        [ snip ]
    drivers/acpi/prmt.c:56: syntax error

The syntax error occurs in the following code in drivers/acpi/prmt.c:

    struct prm_handler_info {
            [ snip ]
            efi_status_t (__efiapi *handler_addr)(u64, void *);
            [ snip ]
    };

The issue arises from __efiapi, which is defined as either
__attribute__((ms_abi)) or __attribute__((regparm(0))).

This commit allows nested_declarator to be prefixed with attributes.

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