[Skeleton] Add missing exports from the barrel (mui#16960)

Author: mejackreed

packages/material-ui-lab/src/index.d.ts

@@ -1,4 +1,5 @@
 export { default as Rating } from './Rating';
+export { default as Skeleton } from './Skeleton';
 export { default as SpeedDial } from './SpeedDial';
 export { default as SpeedDialAction } from './SpeedDialAction';
 export { default as SpeedDialIcon } from './SpeedDialIcon';

packages/material-ui-lab/src/index.js

@@ -1,4 +1,5 @@
 export { default as Rating } from './Rating';
+export { default as Skeleton } from './Skeleton';
 export { default as Slider } from './Slider';
 export { default as SpeedDial } from './SpeedDial';
 export { default as SpeedDialAction } from './SpeedDialAction';