chore(drag-drop): Fix compilation in TS3.6. (#17217)

This version compiles in 3.5 and 3.6.

Author: mprobst

src/cdk/drag-drop/drag-styling.ts

@@ -30,7 +30,7 @@ export function extendStyles(
     source: Partial<DragCSSStyleDeclaration>) {
   for (let key in source) {
     if (source.hasOwnProperty(key)) {
-      dest[key as keyof CSSStyleDeclaration] = source[key as keyof CSSStyleDeclaration];
+      dest[key] = source[key]!;
     }
   }