cocoa.messages: Add a Cocoa union type to import NSData.

Fixes bootstrap for macOS 10.14 for the error:
```
no-objc-type
name "("
```

Fixes #2059.

basis/cocoa/messages/messages.factor

@@ -126,6 +126,7 @@ H{
     { "@" id }
     { "#" Class }
     { ":" SEL }
+    { "(" c:void* }
 }
 cell {
     { 4 [ H{