fix(platform-server): add missing peer dependency for rxjs (angular#60308)

The server package imports from `rxjs`, but the dependency was not
specified as `peerDependency`, nor `dependency`. This surfaces as an
error in strict dependency environments, like with pnpm's symlinked node
modules structure.

This commit fixes this. It presumably doesn't fail with e.g. Yarn, or
npm because of node modules hoisting.

PR Close angular#60308

Author: devversion

packages/platform-server/package.json

@@ -11,7 +11,8 @@
     "@angular/common": "0.0.0-PLACEHOLDER",
     "@angular/compiler": "0.0.0-PLACEHOLDER",
     "@angular/core": "0.0.0-PLACEHOLDER",
-    "@angular/platform-browser": "0.0.0-PLACEHOLDER"
+    "@angular/platform-browser": "0.0.0-PLACEHOLDER",
+    "rxjs": "^6.5.3 || ^7.4.0"
   },
   "dependencies": {
     "tslib": "^2.3.0",