Fix order of @import

In CSS 2.1, any @import rules must precede all other rules (except the
@charset rule, if present).

src/librustdoc/html/static/rustdoc.css

@@ -1,3 +1,5 @@
+@import "normalize.css";
+
 /**
  * Copyright 2013 The Rust Project Developers. See the COPYRIGHT
  * file at the top-level directory of this distribution and at
@@ -59,8 +61,6 @@
     src: local('Source Code Pro Semibold'), url("SourceCodePro-Semibold.woff") format('woff');
 }
 
-@import "normalize.css";
-
 * {
   -webkit-box-sizing: border-box;
      -moz-box-sizing: border-box;