Fix private header name clash in C++HL.

* build.conf (private-includes):
   Add subversion/bindings/cxxhl/src/private/*.hpp.
* subversion/bindings/cxxhl/src/private/exception-private.hpp:
   Renamed from exception.hpp
* subversion/bindings/cxxhl/src/cxxhl-private.hpp:
   Update include directive.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1507415 13f79535-47bb-0310-9956-ffa450edef68