fix(@schematics/angular): providedIn in guard

Author: cyrilletuzi

packages/schematics/angular/guard/files/__name@dasherize__.guard.ts

@@ -2,7 +2,9 @@ import { Injectable } from '@angular/core';
 import { CanActivate, ActivatedRouteSnapshot, RouterStateSnapshot } from '@angular/router';
 import { Observable } from 'rxjs';
 
-@Injectable()
+@Injectable({
+  providedIn: 'root'
+})
 export class <%= classify(name) %>Guard implements CanActivate {
   canActivate(
     next: ActivatedRouteSnapshot,