genksyms: restrict direct-declarator to take one parameter-type-list

Similar to the previous commit, this change makes the parser logic a
little more accurate.

Currently, genksyms accepts the following invalid code:

    struct foo {
            int (*callback)(int)(int)(int);
    };

A direct-declarator should not recursively absorb multiple
( parameter-type-list ) constructs.

In the example above, (*callback) should be followed by at most one
(int).

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