fix mixin definitions inside other mixin definitions

lib/less/tree/ruleset.js

@@ -15,7 +15,7 @@ tree.Ruleset.prototype = {
                 rules.push(rule.evalRules(context));
             } else if (rule instanceof tree.mixin.Call) {
                 Array.prototype.push.apply(rules, rule.eval(context));
-            } else {
+            } else if (! (rule instanceof tree.mixin.Definition)) {
                 rules.push(rule.eval ? rule.eval(context) : '');
             }
         });