Add find_dependency(Protobuf)

[LilyWangLL]

Fix link error:

  The link interface of target "GameNetworkingSockets::GameNetworkingSockets"
  contains:

    protobuf::libprotobuf

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.