singleton: definitions needed (#1428)
singleton: definitions needed (#1428)

* for std::function
* the callback is not declared outside the scope of the Manager class

```
source/common/singleton/manager_impl.cc:10:55: error: 'SingletonFactoryCb' has not been declared
 InstancePtr ManagerImpl::get(const std::string& name, SingletonFactoryCb cb) {
                                                       ^~~~~~~~~~~~~~~~~~
```

Signed-off-by: Vincent Batts <d01a4772f0cffd3f005a7d7f01c59cd104c7a5fe@hashbangbash.com>
dnoe/envoy,envoyproxy/envoy,lizan/envoy,craffert0/envoy,eklitzke/envoy,envoyproxy/envoy,fengli79/envoy,eklitzke/envoy,istio/envoy,lyft/envoy,sebrandon1/envoy,istio/envoy,jrajahalme/envoy,rlazarus/envoy,lizan/envoy,tdmackey/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy-wasm,lyft/envoy,jrajahalme/envoy,envoyproxy/envoy,craffert0/envoy,lizan/envoy,envoyproxy/envoy-wasm,amalgam8/envoy,amalgam8/envoy,eklitzke/envoy,istio/envoy,eklitzke/envoy,envoyproxy/envoy,craffert0/envoy,rlazarus/envoy,lizan/envoy,istio/envoy,envoyproxy/envoy,fengli79/envoy,fengli79/envoy,jrajahalme/envoy,eklitzke/envoy,craffert0/envoy,istio/envoy,sebrandon1/envoy,rlazarus/envoy,dnoe/envoy,sebrandon1/envoy,lyft/envoy,lizan/envoy,sebrandon1/envoy,jrajahalme/envoy,lyft/envoy,amalgam8/envoy,dnoe/envoy,lyft/envoy,rlazarus/envoy,envoyproxy/envoy,lyft/envoy,dnoe/envoy,sebrandon1/envoy,tdmackey/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,jrajahalme/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,tdmackey/envoy,envoyproxy/envoy,istio/envoy,tdmackey/envoy,amalgam8/envoy,dnoe/envoy,eklitzke/envoy,jrajahalme/envoy,envoyproxy/envoy,lizan/envoy,fengli79/envoy,envoyproxy/envoy-wasm,istio/envoy,envoyproxy/envoy-wasm