Add spacing for more space

As noted in #891 at certain viewport, the space between menu and product
name is very tight.

This adds a bit more space between both.

src/components/header/_header.scss

@@ -61,6 +61,7 @@
   .govuk-header__product-name {
     @include govuk-font($size: 24);
     display: inline-table;
+    padding-right: govuk-spacing(2);
   }
 
   .govuk-header__link {