Update mixins.less

Author: febobo

style/mixins.less

@@ -6,7 +6,7 @@
 @import "./mixins/textarea.less";
 @import "./mixins/toast.less";
 @import "./mixins/list.less";
-@import "mixins/nav-bar.less";
+@import "./mixins/nav-bar.less";
 @import "./mixins/drawer.less";
 @import "./mixins/checkbox.less";
 @import "./mixins/radio.less";