http-errors: Add missing constructor

Allows the following to work:
```typescript
import { HttpError } from "http-errors";

let error: HttpError;
console.log(error instanceof HttpError);
```

http-errors/index.d.ts

@@ -23,6 +23,8 @@ declare module 'http-errors' {
             [code: string]: new (msg?: string) => HttpError;
 
             (...args: Array<Error | string | number | Object>): HttpError;
+            
+            HttpError: HttpErrorConstructor;
 
             Continue: HttpErrorConstructor;
             SwitchingProtocols: HttpErrorConstructor;