fix(cdk/a11y): add @angular/platform-browser to peer dependencies (angular#32471)

This commit angular#32412 introduced a new dependency but it was not listed in the peer dependencies. This causes the build to fail when using PNPM as the package cannot be resolved.

Author: alan-agius4

src/cdk/package.json

@@ -47,6 +47,7 @@
   "peerDependencies": {
     "@angular/core": "0.0.0-NG",
     "@angular/common": "0.0.0-NG",
+    "@angular/platform-browser": "0.0.0-NG",
     "rxjs": "^6.5.3 || ^7.4.0"
   },
   "dependencies": {