improve placements type

Author: shaodahong

src/placements.tsx

@@ -1,11 +1,13 @@
+import { BuildInPlacements } from 'rc-trigger';
+
 const autoAdjustOverflow = {
   adjustX: 1,
   adjustY: 1,
 };
 
 const targetOffset = [0, 0];
 
-export const placements = {
+export const placements: BuildInPlacements = {
   left: {
     points: ['cr', 'cl'],
     overflow: autoAdjustOverflow,