Deal with incomplete type as if they are "void"

So..

    struct my_incomplete_type;
    struct mostly_complete {
        int itsanumber;
        char *nameOfTheNumber;
        struct my_incomplete_type *mostly_complete;
    };

will show up as

    struct my_incomplete_type;
    struct mostly_complete {
        int itsanumber;
        char *nameOfTheNumber;
        void *mostly_complete;
    };

which is the same thing, at least from protodump standpoint.

When reading the members of "struct mostly_complete", protodump will try
and read the definition of "struct my_incomplete_type". The problem is
that clang and gcc do not provide it, even if there is a proper
declaration of it somewhere else in your program.

This prevents protodump from trying to figure out if two types relying
on "struct my_incomplete_type" are the same when they are defined in
separate compilation units.