dropdowns.less: Add space before IE8 workaround.

This in turn works around an issue with clean-css (fixed in 3.2.6). So this basically is just a consistency change.

less/dropdowns.less

@@ -11,7 +11,7 @@
   margin-left: 2px;
   vertical-align: middle;
   border-top:   @caret-width-base dashed;
-  border-top:   @caret-width-base ~"solid\9"; // IE8
+  border-top:   @caret-width-base solid ~"\9"; // IE8
   border-right: @caret-width-base solid transparent;
   border-left:  @caret-width-base solid transparent;
 }