Override .custom-select width in .form-inline (twbs#25414)

Fixes twbs#25413

Author: mdo

scss/_forms.scss

@@ -302,7 +302,8 @@ select.form-control-lg {
       display: inline-block;
     }
 
-    .input-group {
+    .input-group,
+    .custom-select {
       width: auto;
     }