Revert the visibility removal of macro_rules visibility (#46)

As discussed in rust-analyzer/ungrammar#46 (comment).

Author: ChayimFriedman2

src/tools/rust-analyzer/lib/ungrammar/rust.ungram

@@ -102,7 +102,7 @@ Item =
 | Use
 
 MacroRules =
-  Attr*
+  Attr* Visibility?
   'macro_rules' '!' Name
   TokenTree