Making the Pool copy constructor explicit means that accidental
copies like that fixed in r1456394 will not happen.

* subversion/bindings/javahl/native/Pool.h
  (Pool::Pool): Add explicit.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1456610 13f79535-47bb-0310-9956-ffa450edef68