Use nesting for .placeholder()

less/forms.less

@@ -92,14 +92,6 @@ output {
   vertical-align: middle;
 }
 
-// Placeholder
-//
-// Placeholder text gets special styles because when browsers invalidate entire
-// lines if it doesn't understand a selector/
-.form-control {
-  .placeholder();
-}
-
 
 // Common form controls
 //
@@ -142,6 +134,12 @@ output {
   // Customize the `:focus` state to imitate native WebKit styles.
   .form-control-focus();
 
+  // Placeholder
+  //
+  // Placeholder text gets special styles because when browsers invalidate entire
+  // lines if it doesn't understand a selector/
+  .placeholder();
+
   // Disabled and read-only inputs
   // Note: HTML5 says that controls under a fieldset > legend:first-child won't
   // be disabled if the fieldset is disabled. Due to implementation difficulty,