Fix a use of visit in rustc::middle::fn_usage

This was working by just skipping an AST node instead of doing the advertised
check. I can't find a test case for it, but this is more correct.

Author: brson

src/comp/middle/fn_usage.rs

@@ -51,7 +51,7 @@ fn fn_usage_expr(expr: @ast::expr,
       ast::expr_call(f, args) {
         let f_ctx = {unsafe_fn_legal: true,
                      generic_bare_fn_legal: true with ctx};
-        visit::visit_expr(f, f_ctx, v);
+        v.visit_expr(f, f_ctx, v);
 
         let args_ctx = {unsafe_fn_legal: false,
                         generic_bare_fn_legal: false with ctx};