Usage of unexposed enums leads to generation of broken bindings

[olbat]

Unexposed enums in struct fields lead to generation of bindings with Void as field's type.
As struct's fields cannot have the Void type, the generated binding is broken.

To reproduce:

$ cat <<'EOF' > /tmp/bug.h
struct st {
	int a;
	enum { x, y } b;
};
struct st *test();
EOF

$ crystal src/main.cr <<'EOF'
@[Include("/tmp/bug.h", prefix: %w(test))]
lib LibTest
end
EOF

lib LibTest
  fun  = test : St*
  struct St
    a : LibC::Int
    b : Void
  end
end

When trying to use the binding, the following error is thrown at compile time: can't use Void as a struct field type.

[olbat]

I've proposed a fix for this in #67.