using @line-height-computed in .navbar-nav link

When a user changed the @line-height-base/@line-height-computed var, the
nav-bar items would not scale appropriately.

less/navbar.less

@@ -28,7 +28,7 @@
     padding-top: ((@navbar-height - @line-height-computed) / 2);
     padding-bottom: ((@navbar-height - @line-height-computed) / 2);
     color: @navbar-link-color;
-    line-height: 20px;
+    line-height: @line-height-computed;
     border-radius: @border-radius-base;
     &:hover,
     &:focus {